BUG: Series.sort_values ignore_index has no effect on already sorted Series (#65833)

doc/source/whatsnew/v3.1.0.rst

@@ -329,7 +329,6 @@ Indexing
 - Fixed bug in :meth:`DataFrame.loc` where assigning an iterable to a single cell in an ``object`` dtype column incorrectly raised a ``ValueError`` (:issue:`57962`)
 - Fixed bug in :meth:`DataFrame.loc` where assigning with duplicate column names and new columns corrupted unrelated columns (:issue:`58317`)
 - Fixed segfault in :meth:`DataFrame.loc` when repeatedly adding new rows to an object-dtype-indexed :class:`DataFrame` (:issue:`21968`)
--
 
 Missing
 ^^^^^^^
@@ -408,6 +407,7 @@ Reshaping
 - Bug in :func:`merge` where merging on a :class:`MultiIndex` containing ``NaN`` values mapped ``NaN`` keys to the last level value instead of ``NaN`` (:issue:`64492`)
 - Bug in :meth:`DataFrame.pivot_table` with ``margins=True`` raising ``TypeError`` when ``values`` has an :class:`ExtensionDtype` that cannot hold ``NA`` (e.g. :class:`IntervalDtype` with an integer subtype) and no ``columns`` were specified (:issue:`55484`)
 - Bug in :meth:`Index.union` where the result could be unsorted when both inputs were monotonic increasing but disjoint, when ``sort`` was not ``False`` (:issue:`54646`)
+- Fixed bug in :meth:`Series.sort_values` where ``ignore_index=True`` had no effect on an already-sorted Series (:issue:`65833`)
 - In :func:`pivot_table`, when ``values`` is empty, the aggregation will be computed on a Series of all NA values (:issue:`46475`)
 -
 

pandas/core/series.py

@@ -4079,9 +4079,12 @@ def sort_values(
         sorted_index = nargsort(values_to_sort, kind, bool(ascending), na_position)
 
         if is_range_indexer(sorted_index, len(sorted_index)):
+            result = self if inplace else self.copy(deep=False)
+            if ignore_index:
+                result.index = default_index(len(sorted_index))
             if inplace:
-                return self._update_inplace(self)
-            return self.copy(deep=False)
+                return None
+            return result
 
         result = self._constructor(
             self._values[sorted_index], index=self.index[sorted_index], copy=False

pandas/tests/series/methods/test_sort_values.py

@@ -179,6 +179,21 @@ def test_sort_values_ignore_index(
         tm.assert_series_equal(result_ser, expected)
         tm.assert_series_equal(ser, Series(original_list))
 
+    def test_sort_values_ignore_index_on_already_sorted(self):
+        # GH 65833 - ignore_index had no effect on already sorted Series
+        ser = Series([1, 2, 3], index=[2, 3, 4])
+        expected = Series([1, 2, 3], index=[0, 1, 2])
+        result = ser.sort_values(ignore_index=True)
+        tm.assert_series_equal(result, expected)
+
+        ser2 = Series([1, 2, 3], index=[2, 3, 4])
+        ser2.sort_values(ignore_index=True, inplace=True)
+        tm.assert_series_equal(ser2, expected)
+
+        ser3 = Series([1, 2, 3], index=[2, 3, 4])
+        result3 = ser3.sort_values(ignore_index=False)
+        tm.assert_series_equal(result3, Series([1, 2, 3], index=[2, 3, 4]))
+
     def test_mergesort_descending_stability(self):
         # GH 28697
         s = Series([1, 2, 1, 3], ["first", "b", "second", "c"])