diff --git a/pandas/tests/groupby/test_groupby.py b/pandas/tests/groupby/test_groupby.py
index 4388913511be2..7db17da9961d7 100644
--- a/pandas/tests/groupby/test_groupby.py
+++ b/pandas/tests/groupby/test_groupby.py
@@ -2710,6 +2710,26 @@ def test_single_element_list_grouping():
     assert result == expected
 
 
+def test_groupby_string_dtype():
+    # GH 40148
+    df = DataFrame({"str_col": ["a", "b", "c", "a"], "num_col": [1, 2, 3, 2]})
+    df["str_col"] = df["str_col"].astype("string")
+    expected = DataFrame(
+        {
+            "str_col": [
+                "a",
+                "b",
+                "c",
+            ],
+            "num_col": [1.5, 2.0, 3.0],
+        }
+    )
+    expected["str_col"] = expected["str_col"].astype("string")
+    grouped = df.groupby("str_col", as_index=False)
+    result = grouped.mean()
+    tm.assert_frame_equal(result, expected)
+
+
 @pytest.mark.parametrize(
     "level_arg, multiindex", [([0], False), ((0,), False), ([0], True), ((0,), True)]
 )