diff --git a/README.md b/README.md
index d34c9b0..134c03e 100644
--- a/README.md
+++ b/README.md
@@ -100,3 +100,4 @@ of the package's `Project.toml` file.
 
 * [Working with a Private Registry and/or Private Repositories](docs/ssh_keys.md)
 * [Registering a Package in a Subdirectory of a Repository](docs/subdir.md)
+* [Merging Registries](docs/merge.md)
diff --git a/docs/merge.md b/docs/merge.md
new file mode 100644
index 0000000..23a466d
--- /dev/null
+++ b/docs/merge.md
@@ -0,0 +1,22 @@
+If you for some reason have two registries that should only be one or
+you want to copy some packages from one registry to another, you can
+use the function `LocalRegistry.merge` (not exported).
+
+    LocalRegistry.merge(target_path, source_path)
+
+Copy all packages in the registry at `source_path` into the
+registry at `target_path`.
+
+    LocalRegistry.merge(target_path, source_path, include = ["A", "B"])
+
+Copy only the packages `A` and `B`.
+
+    LocalRegistry.merge(target_path, source_path, exclude = ["A", "B"])
+
+Copy all packages except `A` and `B`.
+
+----
+
+This gives an error if you try to copy a package that already exists
+in the target repository. There is no built in support for commiting
+or pushing the target registry.
diff --git a/src/LocalRegistry.jl b/src/LocalRegistry.jl
index c00d6e6..874df52 100644
--- a/src/LocalRegistry.jl
+++ b/src/LocalRegistry.jl
@@ -14,12 +14,14 @@ module LocalRegistry
 
 using RegistryTools: RegistryTools, gitcmd, Compress,
                      check_and_update_registry_files, ReturnStatus, haserror,
-                     find_registered_version
+                     find_registered_version, parse_registry,
 using UUIDs: uuid4
 using Pkg: Pkg, TOML
 
 export create_registry, register
 
+include("merge.jl")
+
 # Note: The `uuid` keyword argument is intentionally omitted from the
 # documentation string since it's not intended for users.
 """
diff --git a/src/merge.jl b/src/merge.jl
new file mode 100644
index 0000000..5108165
--- /dev/null
+++ b/src/merge.jl
@@ -0,0 +1,62 @@
+using RegistryTools: write_registry
+
+function merge(target_path::AbstractString,
+               source_path::AbstractString;
+               include::Union{Nothing, Vector{<:AbstractString}} = nothing,
+               exclude::Union{Nothing, Vector{<:AbstractString}} = nothing)
+    if !isnothing(include) && !isnothing(exclude)
+        error("Packages can be either included or excluded, not both.")
+    end
+
+    target_registry = parse_registry(joinpath(target_path, "Registry.toml"))
+    source_registry = parse_registry(joinpath(source_path, "Registry.toml"))
+
+    target_packages = target_registry.packages
+    packages = source_registry.packages
+
+    if !isnothing(include)
+        packages = filter(p -> last(p)["name"] in include, packages)
+        if length(packages) != length(include)
+            missing_packages = setdiff(include, [p["name"] for p in values(packages)])
+            error("Included packages $(missing_packages) do not exist in the source repository")
+        end
+    end
+
+    if !isnothing(exclude)
+        missing_packages = setdiff(exclude, [p["name"] for p in values(packages)])
+        if !isempty(missing_packages)
+            error("Excluded packages $(missing_packages) do not exist in the source repository")
+        end
+        packages = filter(p -> last(p)["name"] ∉ exclude, packages)
+    end
+
+    colliding_names = intersect([p["name"] for p in values(target_packages)],
+                                [p["name"] for p in values(packages)])
+    if !isempty(colliding_names)
+        error("The target registry already contains these packages: $(colliding_names).")
+    end
+
+    colliding_uuids = intersect(keys(target_packages), keys(packages))
+    if !isempty(colliding_uuids)
+        error("The target registry already contains these uuids: $(colliding_uuids).")
+    end
+
+    for (uuid, package) in packages
+        push!(target_packages, uuid => package)
+        package_dir = joinpath(target_path, package["path"])
+        if isdir(package_dir)
+            error("Package dir ", package["path"], " already exists in target registry.")
+        end
+        mkpath(package_dir)
+        for filename in ("Package.toml", "Versions.toml",
+                         "Deps.toml", "Compat.toml")
+            from = joinpath(source_path, package["path"], filename)
+            to = joinpath(target_path, package["path"], filename)
+            if isfile(from)
+                cp(from, to)
+            end
+        end
+    end
+
+    write_registry(joinpath(target_path, "Registry.toml"), target_registry)
+end