diff --git a/rdt/hyper_transformer.py b/rdt/hyper_transformer.py
index 214714e5..e3eee078 100644
--- a/rdt/hyper_transformer.py
+++ b/rdt/hyper_transformer.py
@@ -14,6 +14,7 @@
 from rdt.transformers import (
     BaseTransformer, get_class_by_transformer_name, get_default_transformer,
     get_transformers_by_type)
+from rdt.transformers.utils import flatten_column_list
 
 LOGGER = logging.getLogger(__name__)
 
@@ -25,7 +26,7 @@ def __repr__(self):
         """Pretty print the dictionary."""
         config = {
             'sdtypes': self['sdtypes'],
-            'transformers': {k: repr(v) for k, v in self['transformers'].items()}
+            'transformers': {str(k): repr(v) for k, v in self['transformers'].items()}
         }
 
         printed = json.dumps(config, indent=4)
@@ -115,7 +116,7 @@ def __init__(self):
         self._specified_fields = set()
         self._validate_field_transformers()
         self._valid_output_sdtypes = self._DEFAULT_OUTPUT_SDTYPES
-        self._multi_column_fields = self._create_multi_column_fields()
+        self._multi_column_fields = {}
         self._transformers_sequence = []
         self._output_columns = []
         self._input_columns = []
@@ -205,7 +206,18 @@ def _validate_config(config):
 
         sdtypes = config['sdtypes']
         transformers = config['transformers']
-        if set(sdtypes.keys()) != set(transformers.keys()):
+
+        sdtype_keys = sdtypes.keys()
+        transformer_keys = flatten_column_list(transformers.keys())
+
+        is_transformer_keys_unique = len(transformer_keys) == len(set(transformer_keys))
+        if not is_transformer_keys_unique:
+            raise InvalidConfigError(
+                'Error: Invalid config. Please provide unique keys for the sdtypes '
+                'and transformers.'
+            )
+
+        if set(sdtype_keys) != set(transformer_keys):
             raise InvalidConfigError(
                 "The column names in the 'sdtypes' dictionary must match the "
                 "column names in the 'transformers' dictionary."
@@ -216,10 +228,14 @@ def _validate_config(config):
 
         mismatched_columns = []
         for column_name, transformer in transformers.items():
-            if transformer is not None:
-                sdtype = sdtypes.get(column_name)
+            if transformer is None:
+                continue
+
+            columns = column_name if isinstance(column_name, tuple) else [column_name]
+            for column in columns:
+                sdtype = sdtypes.get(column)
                 if sdtype not in transformer.get_supported_sdtypes():
-                    mismatched_columns.append(column_name)
+                    mismatched_columns.append(column)
 
         if mismatched_columns:
             raise InvalidConfigError(
@@ -519,6 +535,19 @@ def detect_initial_config(self, data):
         LOGGER.info('Config:')
         LOGGER.info(str(config))
 
+    def _get_columns_to_sdtypes(self, field):
+        """Generate the ``columns_to_sdtypes`` dict for the given field.
+
+        Args:
+            field (tuple):
+                Names of the column for the multi column trnasformer.
+        """
+        columns_to_sdtypes = {}
+        for column in field:
+            columns_to_sdtypes[column] = self.field_sdtypes[column]
+
+        return columns_to_sdtypes
+
     def _fit_field_transformer(self, data, field, transformer):
         """Fit a transformer to its corresponding field.
 
@@ -541,7 +570,12 @@ def _fit_field_transformer(self, data, field, transformer):
             self._output_columns.append(field)
 
         else:
-            transformer.fit(data, field)
+            if isinstance(field, tuple):
+                columns_to_sdtypes = self._get_columns_to_sdtypes(field)
+                transformer.fit(data, columns_to_sdtypes)
+            else:
+                transformer.fit(data, field)
+
             self._transformers_sequence.append(transformer)
             data = transformer.transform(data)
 
@@ -609,8 +643,8 @@ def fit(self, data):
         self._validate_detect_config_called(data)
         self._unfit()
         self._input_columns = list(data.columns)
-        for field in self._input_columns:
-            data = self._fit_field_transformer(data, field, self.field_transformers[field])
+        for field_column, field_transformer in self.field_transformers.items():
+            data = self._fit_field_transformer(data, field_column, field_transformer)
 
         self._validate_all_fields_fitted()
         self._fitted = True
diff --git a/rdt/transformers/__init__.py b/rdt/transformers/__init__.py
index 9ac5793a..09f28ae9 100644
--- a/rdt/transformers/__init__.py
+++ b/rdt/transformers/__init__.py
@@ -9,7 +9,7 @@
 from functools import lru_cache
 from pathlib import Path
 
-from rdt.transformers.base import BaseTransformer
+from rdt.transformers.base import BaseMultiColumnTransformer, BaseTransformer
 from rdt.transformers.boolean import BinaryEncoder
 from rdt.transformers.categorical import (
     CustomLabelEncoder, FrequencyEncoder, LabelEncoder, OneHotEncoder, OrderedLabelEncoder,
@@ -22,6 +22,7 @@
 
 __all__ = [
     'BaseTransformer',
+    'BaseMultiColumnTransformer',
     'BinaryEncoder',
     'ClusterBasedNormalizer',
     'CustomLabelEncoder',
diff --git a/rdt/transformers/base.py b/rdt/transformers/base.py
index d43f0c3b..4cf06f32 100644
--- a/rdt/transformers/base.py
+++ b/rdt/transformers/base.py
@@ -362,7 +362,7 @@ def _fit(self, columns_data):
         raise NotImplementedError()
 
     def _set_seed(self, data):
-        hash_value = self.get_input_column()
+        hash_value = self.columns[0]
         for value in data.head(5):
             hash_value += str(value)
 
@@ -479,3 +479,113 @@ def reverse_transform(self, data):
         data = self._add_columns_to_data(data, reversed_data, self.columns)
 
         return data
+
+
+class BaseMultiColumnTransformer(BaseTransformer):
+    """Base class for all multi column transformers.
+
+    The ``BaseMultiColumnTransformer`` class contains methods that must be implemented
+    in order to create a new multi column transformer.
+
+    Attributes:
+        columns_to_sdtypes (dict):
+            Dictionary mapping each column to its sdtype.
+    """
+
+    def __init__(self):
+        super().__init__()
+        self.columns_to_sdtypes = {}
+
+    def get_input_column(self):
+        """Override ``get_input_column`` method from ``BaseTransformer``.
+
+        Raise an error because for multi column transformers, ``get_input_columns``
+        must be used instead.
+        """
+        raise NotImplementedError(
+            'MultiColumnTransformers does not have a single input column.'
+            'Please use ``get_input_columns`` instead.'
+        )
+
+    def get_input_columns(self):
+        """Return input column name for transformer.
+
+        Returns:
+            list:
+                Input column names.
+        """
+        return self.columns
+
+    def _get_prefix(self):
+        """Return the prefix of the output columns.
+
+        Returns:
+            str:
+                Prefix of the output columns.
+        """
+        raise NotImplementedError()
+
+    def _get_output_to_property(self, property_):
+        self.column_prefix = self._get_prefix()
+        output = {}
+        for output_column, properties in self.output_properties.items():
+            # if 'sdtype' is not in the dict, ignore the column
+            if property_ not in properties:
+                continue
+
+            if self.column_prefix is None:
+                output[f'{output_column}'] = properties[property_]
+            else:
+                output[f'{self.column_prefix}.{output_column}'] = properties[property_]
+
+        return output
+
+    def _validate_columns_to_sdtypes(self, data, columns_to_sdtypes):
+        """Check that all the columns in ``columns_to_sdtypes`` are present in the data."""
+        missing = set(columns_to_sdtypes.keys()) - set(data.columns)
+        if missing:
+            missing_to_print = ', '.join(missing)
+            raise ValueError(f'Columns ({missing_to_print}) are not present in the data.')
+
+    def _fit(self, data):
+        """Fit the transformer to the data.
+
+        Args:
+            data (pandas.DataFrame):
+                Data to transform.
+        """
+        raise NotImplementedError()
+
+    @random_state
+    def fit(self, data, columns_to_sdtypes):
+        """Fit the transformer to a ``column`` of the ``data``.
+
+        Args:
+            data (pandas.DataFrame):
+                The entire table.
+            columns_to_sdtypes (dict):
+                Dictionary mapping each column to its sdtype.
+        """
+        self._validate_columns_to_sdtypes(data, columns_to_sdtypes)
+        self.columns_to_sdtypes = columns_to_sdtypes
+        self._store_columns(list(self.columns_to_sdtypes.keys()), data)
+        self._set_seed(data)
+        columns_data = self._get_columns_data(data, self.columns)
+        self._fit(columns_data)
+        self._build_output_columns(data)
+
+    def fit_transform(self, data, columns_to_sdtypes):
+        """Fit the transformer to a `column` of the `data` and then transform it.
+
+        Args:
+            data (pandas.DataFrame):
+                The entire table.
+            columns_to_sdtypes (dict):
+                Dictionary mapping each column to its sdtype.
+
+        Returns:
+            pd.DataFrame:
+                The entire table, containing the transformed data.
+        """
+        self.fit(data, columns_to_sdtypes)
+        return self.transform(data)
diff --git a/rdt/transformers/utils.py b/rdt/transformers/utils.py
index 84a219a8..fe264764 100644
--- a/rdt/transformers/utils.py
+++ b/rdt/transformers/utils.py
@@ -163,3 +163,24 @@ def fill_nan_with_none(data):
             Original data with nan values replaced by None.
     """
     return data.fillna(np.nan).replace([np.nan], [None])
+
+
+def flatten_column_list(column_list):
+    """Flatten a list of columns.
+
+    Args:
+        column_list (list):
+            List of columns to flatten.
+
+    Returns:
+        list:
+            Flattened list of columns.
+    """
+    flattened = []
+    for column in column_list:
+        if isinstance(column, tuple):
+            flattened.extend(column)
+        else:
+            flattened.append(column)
+
+    return flattened
diff --git a/tests/integration/test_hyper_transformer.py b/tests/integration/test_hyper_transformer.py
index bb708109..4fb3967c 100644
--- a/tests/integration/test_hyper_transformer.py
+++ b/tests/integration/test_hyper_transformer.py
@@ -6,12 +6,14 @@
 import pandas as pd
 import pytest
 
-from rdt import HyperTransformer, get_demo
+from rdt import get_demo
 from rdt.errors import ConfigNotSetError, InvalidConfigError, InvalidDataError, NotFittedError
+from rdt.hyper_transformer import Config, HyperTransformer
 from rdt.transformers import (
-    AnonymizedFaker, BaseTransformer, BinaryEncoder, ClusterBasedNormalizer, FloatFormatter,
-    FrequencyEncoder, LabelEncoder, OneHotEncoder, RegexGenerator, UniformEncoder,
-    UnixTimestampEncoder, get_default_transformer, get_default_transformers)
+    AnonymizedFaker, BaseMultiColumnTransformer, BaseTransformer, BinaryEncoder,
+    ClusterBasedNormalizer, FloatFormatter, FrequencyEncoder, LabelEncoder, OneHotEncoder,
+    RegexGenerator, UniformEncoder, UnixTimestampEncoder, get_default_transformer,
+    get_default_transformers)
 from rdt.transformers.datetime import OptimizedTimestampEncoder
 from rdt.transformers.numerical import GaussianNormalizer
 from rdt.transformers.pii.anonymizer import PseudoAnonymizedFaker
@@ -55,6 +57,29 @@ def _reverse_transform(self, data):
 TEST_DATA_INDEX = [4, 6, 3, 8, 'a', 1.0, 2.0, 3.0]
 
 
+class DummyMultiColumnTransformerNumerical(BaseMultiColumnTransformer):
+    """Multi column transformer that takes categorical data."""
+
+    SUPPORTED_SDTYPES = ['categorical', 'boolean']
+
+    def _fit(self, data):
+        self.output_properties = {
+            column: {
+                'sdtype': 'numerical',
+                'next_transformer': None,
+            } for column in self.columns
+        }
+
+    def _get_prefix(self):
+        return None
+
+    def _transform(self, data):
+        return data.astype(float)
+
+    def _reverse_transform(self, data):
+        return data.astype(str)
+
+
 def get_input_data():
     datetimes = pd.to_datetime([
         '2010-02-01',
@@ -1418,3 +1443,40 @@ def test_random_seed(self):
 
         # Assert
         pd.testing.assert_frame_equal(reversed1, reversed2)
+
+    def test_hypertransformer_with_mutli_column_transformer_end_to_end(self):
+        """Test ``HyperTransformer`` with mutli column transformers end to end."""
+        # Setup
+        data_test = pd.DataFrame({
+            'A': ['1.0', '2.0', '3.0'],
+            'B': ['4.0', '5.0', '6.0'],
+            'C': [True, False, True]
+        })
+        dict_config = {
+            'sdtypes': {
+                'A': 'categorical',
+                'B': 'categorical',
+                'C': 'boolean'
+            },
+            'transformers': {
+                ('A', 'B'): DummyMultiColumnTransformerNumerical(),
+                'C': UniformEncoder()
+            }
+        }
+        config = Config(dict_config)
+        ht = HyperTransformer()
+        ht.set_config(config)
+
+        # Run
+        transformed_data = ht.fit_transform(data_test)
+        reverse_transformed_data = ht.reverse_transform(transformed_data)
+
+        # Assert
+        expected_transformed_data = pd.DataFrame({
+            'A': [1.0, 2.0, 3.0],
+            'B': [4.0, 5.0, 6.0],
+            'C': [0.5892351646057272, 0.8615278122985615, 0.36493646501970534]
+        })
+
+        pd.testing.assert_frame_equal(transformed_data, expected_transformed_data)
+        pd.testing.assert_frame_equal(reverse_transformed_data, data_test)
diff --git a/tests/integration/test_transformers.py b/tests/integration/test_transformers.py
index 8a1cf130..5fc4d72a 100644
--- a/tests/integration/test_transformers.py
+++ b/tests/integration/test_transformers.py
@@ -69,7 +69,7 @@ def _is_valid_transformer(transformer_name):
     """Determine if transformer should be tested or not."""
     invalid_names = [
         'IdentityTransformer', 'Dummy', 'OrderedLabelEncoder', 'CustomLabelEncoder',
-        'OrderedUniformEncoder',
+        'OrderedUniformEncoder', 'BaseMultiColumnTransformer'
     ]
     return all(invalid_name not in transformer_name for invalid_name in invalid_names)
 
diff --git a/tests/integration/transformers/test_base.py b/tests/integration/transformers/test_base.py
index de96dfb9..cff48696 100644
--- a/tests/integration/transformers/test_base.py
+++ b/tests/integration/transformers/test_base.py
@@ -3,7 +3,7 @@
 import numpy as np
 import pandas as pd
 
-from rdt.transformers.base import BaseTransformer
+from rdt.transformers.base import BaseMultiColumnTransformer, BaseTransformer
 
 
 def test_dummy_transformer_series_output():
@@ -129,3 +129,177 @@ def _reverse_transform(self, data):
     })
     pd.testing.assert_frame_equal(expected_transform, transformed)
     pd.testing.assert_frame_equal(reverse, data)
+
+
+def test_multi_column_transformer_same_number_of_columns_input_output():
+    """Test a multi-column transformer when the same of input and output columns."""
+    # Setup
+    class AdditionTransformer(BaseMultiColumnTransformer):
+        """This transformer takes 3 columns and return the cumulative sum of each row."""
+        def _fit(self, columns_data):
+            self.output_properties = {
+                f'{self.columns[0]}': {'sdtype': 'numerical'},
+                f'{self.columns[0]}+{self.columns[1]}': {'sdtype': 'numerical'},
+                f'{self.columns[0]}+{self.columns[1]}+{self.columns[2]}': {'sdtype': 'numerical'}
+            }
+
+        def _get_prefix(self):
+            return None
+
+        def _transform(self, data):
+            return data.cumsum(axis=1)
+
+        def _reverse_transform(self, data):
+            result = data.diff(axis=1)
+            result.iloc[:, 0] = data.iloc[:, 0]
+
+            return result.astype('int64')
+
+    data_test = pd.DataFrame({
+        'col_1': [1, 2, 3],
+        'col_2': [10, 20, 30],
+        'col_3': [100, 200, 300]
+    })
+
+    columns_to_sdtypes = {
+        'col_1': 'numerical',
+        'col_2': 'numerical',
+        'col_3': 'numerical'
+    }
+    transformer = AdditionTransformer()
+
+    # Run
+    transformed = transformer.fit_transform(data_test, columns_to_sdtypes)
+    reverse = transformer.reverse_transform(transformed)
+
+    # Assert
+    expected_transform = pd.DataFrame({
+        'col_1': [1, 2, 3],
+        'col_1+col_2': [11, 22, 33],
+        'col_1+col_2+col_3': [111, 222, 333]
+    })
+    pd.testing.assert_frame_equal(expected_transform, transformed)
+    pd.testing.assert_frame_equal(reverse, data_test)
+
+
+def test_multi_column_transformer_less_output_than_input_columns():
+    """Test a multi-column transformer when the output has less columns than the input."""
+    class ConcatenateTransformer(BaseMultiColumnTransformer):
+        """This transformer takes 4 columns and concatenate them into 2 columns.
+        The two first and last columns are concatenated together.
+        """
+        def _fit(self, columns_data):
+            self.name_1 = self.columns[0] + '#' + self.columns[1]
+            self.name_2 = self.columns[2] + '#' + self.columns[3]
+            self.output_properties = {
+                f'{self.name_1}.concatenate_1': {'sdtype': 'categorical'},
+                f'{self.name_2}.concatenate_2': {'sdtype': 'categorical'}
+            }
+
+        def _get_prefix(self):
+            return None
+
+        def _transform(self, data):
+            data[self.name_1] = data.iloc[:, 0] + '#' + data.iloc[:, 1]
+            data[self.name_2] = data.iloc[:, 2] + '#' + data.iloc[:, 3]
+
+            return data.drop(columns=self.columns)
+
+        def _reverse_transform(self, data):
+            result = data.copy()
+            column_names = list(data.columns)
+
+            col1, col2 = column_names[0].split('#')
+            result[[col1, col2]] = result[column_names[0]].str.split('#', expand=True)
+
+            col3, col4 = column_names[1].split('#')
+            result[[col3, col4]] = result[column_names[1]].str.split('#', expand=True)
+
+            return result.drop(columns=column_names)
+
+    data_test = pd.DataFrame({
+        'col_1': ['A', 'B', 'C'],
+        'col_2': ['D', 'E', 'F'],
+        'col_3': ['G', 'H', 'I'],
+        'col_4': ['J', 'K', 'L']
+    })
+
+    columns_to_sdtypes = {
+        'col_1': 'categorical',
+        'col_2': 'categorical',
+        'col_3': 'categorical',
+        'col_4': 'categorical'
+    }
+    transformer = ConcatenateTransformer()
+
+    # Run
+    transformer.fit(data_test, columns_to_sdtypes)
+    transformed = transformer.transform(data_test)
+    reverse = transformer.reverse_transform(transformed)
+
+    # Assert
+    expected_transform = pd.DataFrame({
+        'col_1#col_2.concatenate_1': ['A#D', 'B#E', 'C#F'],
+        'col_3#col_4.concatenate_2': ['G#J', 'H#K', 'I#L']
+    })
+    pd.testing.assert_frame_equal(expected_transform, transformed)
+    pd.testing.assert_frame_equal(reverse, data_test)
+
+
+def test_multi_column_transformer_more_output_than_input_columns():
+    """Test a multi-column transformer when the output has more columns than the input."""
+    class ExpandTransformer(BaseMultiColumnTransformer):
+
+        def _fit(self, columns_data):
+            self.output_properties = {
+                f'{self.columns[0]}.first_part_1': {'sdtype': 'categorical'},
+                f'{self.columns[0]}.second_part_1': {'sdtype': 'categorical'},
+                f'{self.columns[1]}.first_part_2': {'sdtype': 'categorical'},
+                f'{self.columns[1]}.second_part_2': {'sdtype': 'categorical'}
+            }
+
+        def _get_prefix(self):
+            return None
+
+        def _transform(self, data):
+            data[self.output_columns[0]] = data[self.columns[0]].str[0]
+            data[self.output_columns[1]] = data[self.columns[0]].str[1]
+            data[self.output_columns[2]] = data[self.columns[1]].str[0]
+            data[self.output_columns[3]] = data[self.columns[1]].str[1]
+
+            return data.drop(columns=self.columns)
+
+        def _reverse_transform(self, data):
+            result = data.copy()
+            reverse_1 = result[self.output_columns[0]] + result[self.output_columns[1]]
+            reverse_2 = result[self.output_columns[2]] + result[self.output_columns[3]]
+            result[self.columns[0]] = reverse_1
+            result[self.columns[1]] = reverse_2
+
+            return result.drop(columns=self.output_columns)
+
+    data_test = pd.DataFrame({
+        'col_1': ['AB', 'CD', 'EF'],
+        'col_2': ['GH', 'IJ', 'KL'],
+    })
+
+    columns_to_sdtypes = {
+        'col_1': 'categorical',
+        'col_2': 'categorical'
+    }
+    transformer = ExpandTransformer()
+
+    # Run
+    transformer.fit(data_test, columns_to_sdtypes)
+    transformed = transformer.transform(data_test)
+    reverse = transformer.reverse_transform(transformed)
+
+    # Assert
+    expected_transform = pd.DataFrame({
+        'col_1.first_part_1': ['A', 'C', 'E'],
+        'col_1.second_part_1': ['B', 'D', 'F'],
+        'col_2.first_part_2': ['G', 'I', 'K'],
+        'col_2.second_part_2': ['H', 'J', 'L']
+    })
+    pd.testing.assert_frame_equal(expected_transform, transformed)
+    pd.testing.assert_frame_equal(reverse, data_test)
diff --git a/tests/unit/test_hyper_transformer.py b/tests/unit/test_hyper_transformer.py
index dc31bc8a..b9e0924f 100644
--- a/tests/unit/test_hyper_transformer.py
+++ b/tests/unit/test_hyper_transformer.py
@@ -11,8 +11,8 @@
     ConfigNotSetError, InvalidConfigError, InvalidDataError, NotFittedError, TransformerInputError,
     TransformerProcessingError)
 from rdt.transformers import (
-    AnonymizedFaker, BinaryEncoder, FloatFormatter, FrequencyEncoder, LabelEncoder, RegexGenerator,
-    UnixTimestampEncoder)
+    AnonymizedFaker, BaseMultiColumnTransformer, BinaryEncoder, FloatFormatter, FrequencyEncoder,
+    LabelEncoder, RegexGenerator, UnixTimestampEncoder)
 from rdt.transformers.base import BaseTransformer
 from rdt.transformers.numerical import ClusterBasedNormalizer
 
@@ -99,9 +99,8 @@ def test__validate_field_transformers(self):
         with pytest.raises(ValueError, match=error_msg):
             ht._validate_field_transformers()
 
-    @patch('rdt.hyper_transformer.HyperTransformer._create_multi_column_fields')
     @patch('rdt.hyper_transformer.HyperTransformer._validate_field_transformers')
-    def test___init__(self, validation_mock, multi_column_mock):
+    def test___init__(self, validation_mock):
         """Test create new instance of HyperTransformer"""
         # Run
         ht = HyperTransformer()
@@ -109,6 +108,7 @@ def test___init__(self, validation_mock, multi_column_mock):
         # Asserts
         assert ht.field_sdtypes == {}
         assert ht.field_transformers == {}
+        assert ht._multi_column_fields == {}
         assert ht._specified_fields == set()
         assert ht._valid_output_sdtypes == ht._DEFAULT_OUTPUT_SDTYPES
         assert ht._transformers_sequence == []
@@ -117,7 +117,6 @@ def test___init__(self, validation_mock, multi_column_mock):
         assert ht._fitted_fields == set()
         assert ht._fitted is False
         assert ht._modified_config is False
-        multi_column_mock.assert_called_once()
         validation_mock.assert_called_once()
 
     def test__unfit(self):
@@ -327,6 +326,30 @@ def test_detect_initial_config(self, logger_mock):
             call(expected_config)
         ])
 
+    def test__get_columns_to_sdtypes(self):
+        """Test the ``_get_columns_to_sdtypes`` method."""
+        # Setup
+        ht = HyperTransformer()
+        ht.field_sdtypes = {
+            'col1': 'numerical',
+            'col2': 'categorical',
+            'col3': 'boolean',
+            'col4': 'datetime',
+        }
+
+        column_tuple = ('col1', 'col2', 'col3')
+
+        # Run
+        columns_to_sdtypes = ht._get_columns_to_sdtypes(column_tuple)
+
+        # Assert
+        expected_columns_to_sdtypes = {
+            'col1': 'numerical',
+            'col2': 'categorical',
+            'col3': 'boolean',
+        }
+        assert columns_to_sdtypes == expected_columns_to_sdtypes
+
     def test__fit_field_transformer(self):
         """Test the ``_fit_field_transformer`` method.
 
@@ -480,6 +503,32 @@ def test__validate_config(self):
         with pytest.raises(InvalidConfigError, match=error_msg):
             HyperTransformer._validate_config(config)
 
+    def test_validate_config_not_unique_field(self):
+        """Test the ``_validate_config`` method when a column is repeated in a field."""
+        # Setup
+        transformers = {
+            'column1': FloatFormatter(),
+            'column2': FrequencyEncoder(),
+            ('column2', 'column3'): None
+        }
+        sdtypes = {
+            'column1': 'numerical',
+            'column2': 'numerical',
+            'column3': 'numerical'
+        }
+        config = {
+            'sdtypes': sdtypes,
+            'transformers': transformers
+        }
+
+        # Run
+        error_msg = re.escape(
+            'Error: Invalid config. Please provide unique keys for the sdtypes '
+            'and transformers.'
+        )
+        with pytest.raises(InvalidConfigError, match=error_msg):
+            HyperTransformer._validate_config(config)
+
     @patch('rdt.hyper_transformer.warnings')
     def test__validate_config_no_warning(self, warnings_mock):
         """Test the ``_validate_config`` method with no warning.
@@ -499,11 +548,13 @@ def test__validate_config_no_warning(self, warnings_mock):
         # Setup
         transformers = {
             'column1': FloatFormatter(),
-            'column2': FrequencyEncoder()
+            'column2': FrequencyEncoder(),
+            'column3': None
         }
         sdtypes = {
             'column1': 'numerical',
-            'column2': 'categorical'
+            'column2': 'categorical',
+            'column3': 'numerical'
         }
         config = {
             'sdtypes': sdtypes,
@@ -971,6 +1022,56 @@ def test_fit(self):
         ht._validate_detect_config_called.assert_called_once()
         ht._unfit.assert_called_once()
 
+    def test_fit_with_multi_column_transformer(self):
+        """Test the ``fit`` method with a multi-column transformer."""
+        # Setup
+        class MultiColumnTransformer(BaseMultiColumnTransformer):
+            def _fit(self, data):
+                self.output_properties = {
+                    column: {'next_transformer': None, 'sdtype': 'numerical'}
+                    for column in self.columns
+                }
+
+            def _get_prefix(self):
+                return None
+
+            def _transform(self, data):
+                return data
+
+            def _reverse_transform(self, data):
+                return data
+
+        field_transformers = {
+            ('col1', 'col2'): MultiColumnTransformer(),
+            'col3': FloatFormatter()
+        }
+        field_sdtypes = {
+            'col1': 'numerical',
+            'col2': 'categorical',
+            'col3': 'numerical'
+        }
+
+        columns_to_sdtype = {
+            'col1': 'numerical',
+            'col2': 'categorical',
+        }
+        ht = HyperTransformer()
+        ht.field_transformers = field_transformers
+        ht.field_sdtypes = field_sdtypes
+        ht._get_columns_to_sdtypes = Mock(return_value=columns_to_sdtype)
+
+        data = pd.DataFrame({
+            'col1': [1, 2, 3],
+            'col2': ['a', 'b', 'c'],
+            'col3': [1, 2, 3]
+        })
+
+        # Run
+        ht.fit(data)
+
+        # Assert
+        ht._get_columns_to_sdtypes.assert_called_once()
+
     def test_fit_warns(self):
         """Test it warns when different transformer instances produce the same column name.
 
diff --git a/tests/unit/transformers/test_base.py b/tests/unit/transformers/test_base.py
index edcfe799..f9a8fc11 100644
--- a/tests/unit/transformers/test_base.py
+++ b/tests/unit/transformers/test_base.py
@@ -7,7 +7,7 @@
 import pytest
 
 from rdt.errors import TransformerInputError
-from rdt.transformers import BaseTransformer, NullTransformer
+from rdt.transformers import BaseMultiColumnTransformer, BaseTransformer, NullTransformer
 from rdt.transformers.base import random_state, set_random_states
 
 
@@ -1272,3 +1272,221 @@ def _reverse_transform(self, data):
             'b': [0.0, 0.0, 0.0],
         })
         pd.testing.assert_frame_equal(transformed_data, expected_transformed)
+
+
+class TestBaseMultiColumnTransformer:
+
+    def test___init__(self):
+        """Test the ``__init__`` method."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+
+        # Assert
+        assert transformer.columns_to_sdtypes == {}
+
+    def test_get_input_column(self):
+        """Test the ``get_input_column`` method.
+
+        When the ``get_input_column`` method is called, it should raise a ``NotImplementedError``.
+        """
+        # Setup
+        expected_message = (
+            'MultiColumnTransformers does not have a single input column.'
+            'Please use ``get_input_columns`` instead.'
+        )
+
+        # Run and Assert
+        with pytest.raises(NotImplementedError, match=expected_message):
+            BaseMultiColumnTransformer().get_input_column()
+
+    def test_get_input_columns(self):
+        """Test the ``get_input_columns`` method."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+        transformer.columns = ['a', 'b', 'c']
+
+        # Run
+        output = transformer.get_input_columns()
+
+        # Assert
+        assert output == ['a', 'b', 'c']
+
+    def test__get_prefixes(self):
+        """Test the ``_get_prefix`` method."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+
+        # Run and Assert
+        with pytest.raises(NotImplementedError):
+            transformer._get_prefix()
+
+    def test__get_output_to_property(self):
+        """Test the ``_get_output_to_property`` method."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+        transformer.output_properties = {
+            'col_1': {'sdtype': 'numerical'},
+            'col_2': {'sdtype': 'categorical'},
+            'col_3': {'next_transformer': None},
+        }
+        transformer._get_prefix = Mock(return_value='prefix')
+
+        # Run
+        output = transformer._get_output_to_property('sdtype')
+
+        # Assert
+        expected_output = {
+            'prefix.col_1': 'numerical',
+            'prefix.col_2': 'categorical',
+        }
+        assert output == expected_output
+        transformer._get_prefix.assert_called_once_with()
+
+    def test__get_output_to_property_with_single_prefix(self):
+        """Test the ``_get_output_to_property`` method when the prefix is a single string."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+        transformer.output_properties = {
+            'col_1': {'sdtype': 'numerical'},
+            'col_2': {'sdtype': 'categorical'},
+            'col_3': {'sdtype': 'boolean'},
+        }
+        prefixes = 'prefix'
+        transformer._get_prefix = Mock(return_value=prefixes)
+
+        # Run
+        output = transformer._get_output_to_property('sdtype')
+
+        # Assert
+        expected_output = {
+            'prefix.col_1': 'numerical',
+            'prefix.col_2': 'categorical',
+            'prefix.col_3': 'boolean',
+        }
+        assert output == expected_output
+        transformer._get_prefix.assert_called_once_with()
+
+    def test__get_output_to_property_with_prefix_none(self):
+        """Test the ``_get_output_to_property`` method when the prefix is None."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+        transformer.output_properties = {
+            'col_1': {'sdtype': 'numerical'},
+            'col_2': {'sdtype': 'categorical'},
+            'col_3': {'sdtype': 'boolean'},
+        }
+        prefixes = None
+        transformer._get_prefix = Mock(return_value=prefixes)
+
+        # Run
+        output = transformer._get_output_to_property('sdtype')
+
+        # Assert
+        expected_output = {
+            'col_1': 'numerical',
+            'col_2': 'categorical',
+            'col_3': 'boolean',
+        }
+        assert output == expected_output
+        transformer._get_prefix.assert_called_once_with()
+
+    def test__validate_columns_to_sdtypes(self):
+        """Test the ``_validate_columns_to_sdtypes`` method.
+
+        Test that an error with the missing column names is raised.
+        """
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+        data = pd.DataFrame({
+            'a': [1, 2, 3],
+            'b': ['a', 'b', 'c'],
+            'c': [True, False, True],
+        })
+        columns_to_sdtypes = {
+            'a': 'numerical',
+            'b': 'categorical',
+            'c': 'boolean',
+        }
+
+        # Run and Assert
+        transformer._validate_columns_to_sdtypes(data, columns_to_sdtypes)
+
+        wrong_columns_to_sdtypes = {
+            'a': 'numerical',
+            'b': 'categorical',
+            'd': 'boolean',
+        }
+        expected_error_msg = re.escape(
+            'Columns (d) are not present in the data.'
+        )
+        with pytest.raises(ValueError, match=expected_error_msg):
+            transformer._validate_columns_to_sdtypes(data, wrong_columns_to_sdtypes)
+
+    def test__fit(self):
+        """Test the ``_fit`` method.
+
+        Check that an error is raised when the ``_fit`` method is called.
+        """
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+
+        # Run and Assert
+        with pytest.raises(NotImplementedError):
+            transformer._fit(None)
+
+    def test_fit(self):
+        """Test the ``fit`` method."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+        data = Mock()
+        data_transformer = pd.DataFrame({
+            'a': [1, 2, 3],
+            'b': ['a', 'b', 'c'],
+        })
+        columns_to_sdtypes = {
+            'a': 'numerical',
+            'b': 'categorical',
+        }
+        transformer.columns = ['a', 'b']
+
+        transformer._validate_columns_to_sdtypes = Mock()
+        transformer._store_columns = Mock()
+        transformer._get_columns_data = Mock(return_value=data_transformer)
+        transformer._set_seed = Mock()
+        transformer._fit = Mock()
+        transformer._build_output_columns = Mock()
+
+        # Run
+        transformer.fit(data, columns_to_sdtypes)
+
+        # Assert
+        transformer._validate_columns_to_sdtypes.assert_called_once_with(data, columns_to_sdtypes)
+        transformer._store_columns.assert_called_once_with(
+            ['a', 'b'], data
+        )
+        transformer._set_seed.assert_called_once_with(data)
+        transformer._get_columns_data.assert_called_once_with(data, ['a', 'b'])
+        transformer._fit.assert_called_once_with(data_transformer)
+        transformer._build_output_columns.assert_called_once_with(data)
+
+    def test_fit_transform(self):
+        """Test the ``fit_transform`` method."""
+        # Setup
+        transformer = BaseMultiColumnTransformer()
+        columns_to_sdtypes = ('a', 'b', 'c')
+        data = pd.DataFrame({
+            'a': [1, 2, 3],
+            'b': ['a', 'b', 'c'],
+        })
+        transformer.columns = ['a', 'b']
+        mock_fit = Mock()
+        mock_transform = Mock(return_value=data)
+        transformer.fit = mock_fit
+        transformer.transform = mock_transform
+
+        # Run
+        transformer.fit_transform(data, columns_to_sdtypes)
+
+        # Assert
+        mock_fit.assert_called_once_with(data, columns_to_sdtypes)
+        mock_transform.assert_called_once_with(data)
diff --git a/tests/unit/transformers/test_utils.py b/tests/unit/transformers/test_utils.py
index 2d6eceb0..475a32da 100644
--- a/tests/unit/transformers/test_utils.py
+++ b/tests/unit/transformers/test_utils.py
@@ -1,7 +1,7 @@
 import sre_parse
 from sre_constants import MAXREPEAT
 
-from rdt.transformers.utils import _any, _max_repeat, strings_from_regex
+from rdt.transformers.utils import _any, _max_repeat, flatten_column_list, strings_from_regex
 
 
 def test_strings_from_regex_literal():
@@ -53,3 +53,16 @@ def test_strings_from_regex_very_large_regex():
 
     assert size == 173689027553046619421110743915454114823342474255318764491341273608665169920
     [next(generator) for _ in range(100_000)]
+
+
+def test_flatten_column_list():
+    """Test `flatten_column_list` function."""
+    # Setup
+    column_list = ['column1', ('column2', 'column3'), 'column4', ('column5',), 'column6']
+
+    # Run
+    flattened_list = flatten_column_list(column_list)
+
+    # Assert
+    expected_flattened_list = ['column1', 'column2', 'column3', 'column4', 'column5', 'column6']
+    assert flattened_list == expected_flattened_list