diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 27b331babe692..4220be278572c 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -1283,6 +1283,10 @@ def _validate(self, validate: str):
                 raise MergeError(
                     "Merge keys are not unique in left dataset; not a one-to-many merge"
                 )
+            elif right_unique:
+                raise MergeError(
+                    "Merge keys are unique in right dataset; not a one-to-many merge"
+                )
 
         elif validate in ["many_to_one", "m:1"]:
             if not right_unique:
@@ -1290,9 +1294,20 @@ def _validate(self, validate: str):
                     "Merge keys are not unique in right dataset; "
                     "not a many-to-one merge"
                 )
+            elif left_unique:
+                raise MergeError(
+                    "Merge keys are unique in left dataset; not a many-to-one merge"
+                )
 
         elif validate in ["many_to_many", "m:m"]:
-            pass
+            if left_unique:
+                raise MergeError(
+                    "Merge keys are unique in left dataset; not a many-to-many merge"
+                )
+            elif right_unique:
+                raise MergeError(
+                    "Merge keys are unique in right dataset; not a many-to-many merge"
+                )
 
         else:
             raise ValueError("Not a valid argument for validate")