diff --git a/pandas/tests/groupby/test_transform.py b/pandas/tests/groupby/test_transform.py
index d3972e6ba9008..2093fd28291b2 100644
--- a/pandas/tests/groupby/test_transform.py
+++ b/pandas/tests/groupby/test_transform.py
@@ -1074,3 +1074,32 @@ def test_transform_lambda_with_datetimetz():
         name="time",
     )
     assert_series_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "inputDF, expectedDF",
+    [
+        (
+            DataFrame(
+                {
+                    "A": [121, 121, 121, 121, 231, 231, 676],
+                    "B": [1, 2, np.nan, 3, 3, np.nan, 4],
+                }
+            ),
+            1,
+        ),
+        (
+            DataFrame(
+                {
+                    "A": [121, 121, 121, 121, 231, 231, 676],
+                    "B": [1.0, 2.0, 2.0, 3.0, 3.0, 3.0, 4.0],
+                }
+            ),
+            1,
+        ),
+    ],
+)
+def test_groupby_transform_fillna(inputDF, expectedDF):
+    # GH 27905 - Test fillna in groupby.transform
+    input1 = inputDF.groupby("A").transform(lambda x: x.fillna(x.mean()))
+    assert all(input1 == expectedDF)