BUG: Series rolling count ignores min_periods

doc/source/whatsnew/v1.0.0.rst

@@ -1112,6 +1112,7 @@ Groupby/resample/rolling
 - Bug in :meth:`DataFrame.groupby` when using nunique on axis=1 (:issue:`30253`)
 - Bug in :meth:`GroupBy.quantile` with multiple list-like q value and integer column names (:issue:`30289`)
 - Bug in :meth:`GroupBy.pct_change` and :meth:`core.groupby.SeriesGroupBy.pct_change` causes ``TypeError`` when ``fill_method`` is ``None`` (:issue:`30463`)
+- Bug in :meth:`Rolling.count` and :meth:`Expanding.count` argument ``min_periods`` ignored (:issue:`26996`)
 
 Reshaping
 ^^^^^^^^^

pandas/core/window/rolling.py

@@ -1185,14 +1185,16 @@ def count(self):
 
         window = self._get_window()
         window = min(window, len(obj)) if not self.center else window
+        min_periods = self.min_periods if self.min_periods is not None else 0
+        min_periods = min(min_periods, len(obj)) if not self.center else min_periods
 
         results = []
         for b in blocks:
             result = b.notna().astype(int)
             result = self._constructor(
                 result,
                 window=window,
-                min_periods=0,
+                min_periods=min_periods,
                 center=self.center,
                 axis=self.axis,
                 closed=self.closed,

pandas/tests/window/test_expanding.py

@@ -113,3 +113,10 @@ def test_expanding_axis(self, axis_frame):
 
         result = df.expanding(3, axis=axis_frame).sum()
         tm.assert_frame_equal(result, expected)
+
+
+def test_expanding_count_with_min_periods():
+    # GH 26996
+    result = Series(range(5)).expanding(min_periods=3).count()
+    expected = Series([np.nan, np.nan, 3.0, 4.0, 5.0])
+    tm.assert_series_equal(result, expected)

pandas/tests/window/test_rolling.py

@@ -426,3 +426,10 @@ def test_min_periods1():
     result = df["a"].rolling(3, center=True, min_periods=1).max()
     expected = pd.Series([1.0, 2.0, 2.0, 2.0, 1.0], name="a")
     tm.assert_series_equal(result, expected)
+
+
+def test_rolling_count_with_min_periods():
+    # GH 26996
+    result = Series(range(5)).rolling(3, min_periods=3).count()
+    expected = Series([np.nan, np.nan, 3.0, 3.0, 3.0])
+    tm.assert_series_equal(result, expected)