diff --git a/pandas/tests/util/test_assert_series_equal.py b/pandas/tests/util/test_assert_series_equal.py
index 2209bed67325c..4e5e45b315fc1 100644
--- a/pandas/tests/util/test_assert_series_equal.py
+++ b/pandas/tests/util/test_assert_series_equal.py
@@ -382,3 +382,21 @@ def test_assert_series_equal_identical_na(nulls_fixture):
     # while we're here do Index too
     idx = pd.Index(ser)
     tm.assert_index_equal(idx, idx.copy(deep=True))
+
+
+@pytest.mark.parametrize(
+    "test, constant",
+    [
+        ({"a": [1, 2, 3], "b": [1, 1, 1]}, {"a": [1, 2, 3], "b": 1}),
+        ({"a": [2, 2, 2], "b": [1, 1, 1]}, {"a": 2, "b": 1}),
+    ],
+)
+def test_unique_agg_type_is_series(test, constant):
+    # GH#22558
+    df1 = DataFrame(test)
+    expected = Series(data=constant, index=["a", "b"], dtype="object")
+    aggregation = {"a": "unique", "b": "unique"}
+
+    result = df1.agg(aggregation)
+
+    tm.assert_series_equal(result, expected)