diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 18ee1ad9bcd96..7b564d55a342c 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -260,6 +260,8 @@
 _merge_doc = """
 Merge DataFrame or named Series objects with a database-style join.
 
+A named Series object is treated as a DataFrame with a single named column.
+
 The join is done on columns or indexes. If joining columns on
 columns, the DataFrame indexes *will be ignored*. Otherwise if joining indexes
 on indexes or indexes on a column or columns, the index will be passed on.
diff --git a/pandas/core/reshape/merge.py b/pandas/core/reshape/merge.py
index 4791bbf0ba7f7..be9eeb64bc35d 100644
--- a/pandas/core/reshape/merge.py
+++ b/pandas/core/reshape/merge.py
@@ -27,7 +27,6 @@
     ArrayLike,
     DtypeObj,
     FrameOrSeries,
-    FrameOrSeriesUnion,
     IndexLabel,
     Suffixes,
 )
@@ -81,15 +80,18 @@
 from pandas.core.sorting import is_int64_overflow_possible
 
 if TYPE_CHECKING:
-    from pandas import DataFrame
+    from pandas import (
+        DataFrame,
+        Series,
+    )
     from pandas.core.arrays import DatetimeArray
 
 
-@Substitution("\nleft : DataFrame")
+@Substitution("\nleft : DataFrame or named Series")
 @Appender(_merge_doc, indents=0)
 def merge(
-    left: FrameOrSeriesUnion,
-    right: FrameOrSeriesUnion,
+    left: DataFrame | Series,
+    right: DataFrame | Series,
     how: str = "inner",
     on: IndexLabel | None = None,
     left_on: IndexLabel | None = None,
@@ -322,8 +324,8 @@ def _merger(x, y) -> DataFrame:
 
 
 def merge_asof(
-    left: DataFrame,
-    right: DataFrame,
+    left: DataFrame | Series,
+    right: DataFrame | Series,
     on: IndexLabel | None = None,
     left_on: IndexLabel | None = None,
     right_on: IndexLabel | None = None,
@@ -362,8 +364,8 @@ def merge_asof(
 
     Parameters
     ----------
-    left : DataFrame
-    right : DataFrame
+    left : DataFrame or named Series
+    right : DataFrame or named Series
     on : label
         Field name to join on. Must be found in both DataFrames.
         The data MUST be ordered. Furthermore this must be a numeric column,
@@ -608,8 +610,8 @@ class _MergeOperation:
 
     def __init__(
         self,
-        left: FrameOrSeriesUnion,
-        right: FrameOrSeriesUnion,
+        left: DataFrame | Series,
+        right: DataFrame | Series,
         how: str = "inner",
         on: IndexLabel | None = None,
         left_on: IndexLabel | None = None,
@@ -1599,8 +1601,8 @@ class _OrderedMerge(_MergeOperation):
 
     def __init__(
         self,
-        left: DataFrame,
-        right: DataFrame,
+        left: DataFrame | Series,
+        right: DataFrame | Series,
         on: IndexLabel | None = None,
         left_on: IndexLabel | None = None,
         right_on: IndexLabel | None = None,
@@ -1704,8 +1706,8 @@ class _AsOfMerge(_OrderedMerge):
 
     def __init__(
         self,
-        left: DataFrame,
-        right: DataFrame,
+        left: DataFrame | Series,
+        right: DataFrame | Series,
         on: IndexLabel | None = None,
         left_on: IndexLabel | None = None,
         right_on: IndexLabel | None = None,