diff --git a/api/v3/Contribution/Transact.php b/api/v3/Contribution/Transact.php
index d97167e7f290..137cce175f17 100644
--- a/api/v3/Contribution/Transact.php
+++ b/api/v3/Contribution/Transact.php
@@ -46,6 +46,8 @@ function _civicrm_api3_contribution_transact_spec(&$params) {
 /**
  * Process a transaction and record it against the contact.
  *
+ * @deprecated
+ *
  * @param array $params
  *   Input parameters.
  *
@@ -53,6 +55,7 @@ function _civicrm_api3_contribution_transact_spec(&$params) {
  *   contribution of created or updated record (or a civicrm error)
  */
 function civicrm_api3_contribution_transact($params) {
+  CRM_Core_Error::deprecatedFunctionWarning('The contibution.transact api is unsupported & known to have issues. Please see the section at the bottom of https://docs.civicrm.org/dev/en/latest/financial/OrderAPI/ for getting off it');
   // Set some params specific to payment processing
   // @todo - fix this function - none of the results checked by civicrm_error would ever be an array with
   // 'is_error' set