diff --git a/src/Mod/Part/App/TopoShapeExpansion.cpp b/src/Mod/Part/App/TopoShapeExpansion.cpp
index 2fcd2a890f..9492107fc3 100644
--- a/src/Mod/Part/App/TopoShapeExpansion.cpp
+++ b/src/Mod/Part/App/TopoShapeExpansion.cpp
@@ -3990,9 +3990,10 @@ TopoShape& TopoShape::makeElementMirror(const TopoShape& shape, const gp_Ax2& ax
     }
     gp_Trsf mat;
     mat.SetMirror(ax2);
-    TopLoc_Location loc = shape.getShape().Location();
-    gp_Trsf placement = loc.Transformation();
-    mat = placement * mat;
+    // Note: Do NOT extract and pre-multiply the shape's Location/Placement here.
+    // BRepBuilderAPI_Transform correctly handles shapes with Location already.
+    // Pre-multiplying would double-apply the placement, causing incorrect results
+    // for shapes with non-identity Placement. See GitHub issue #20834.
     BRepBuilderAPI_Transform mkTrf(shape.getShape(), mat);
     return makeElementShape(mkTrf, shape, op);
 }
diff --git a/src/Mod/Part/parttests/TopoShapeTest.py b/src/Mod/Part/parttests/TopoShapeTest.py
index 0cc78f25e7..0ad82fa195 100644
--- a/src/Mod/Part/parttests/TopoShapeTest.py
+++ b/src/Mod/Part/parttests/TopoShapeTest.py
@@ -607,6 +607,76 @@ class TopoShapeTest(unittest.TestCase, TopoShapeAssertions):
         if mirror.ElementMapVersion != "":  # Should be '4' as of Mar 2023.
             self.assertEqual(mirror.ElementMapSize, 26)
 
+    def testTopoShapeMirrorWithPlacement(self):
+        """Test that mirror() produces identical results regardless of how the
+        shape is positioned - via direct coordinates or via Placement.
+        Regression test for GitHub issue #20834.
+
+        The bug was: when a shape has a non-identity Location (Placement),
+        the mirror result was incorrect because the placement was being
+        double-applied in makeElementMirror().
+        """
+        # Create two identical boxes at the same visual location using different methods:
+        # Method 1: Box geometry positioned directly at (0, 30, 0), identity Placement
+        box_direct = Part.makeBox(10, 20, 30, App.Vector(0, 30, 0))
+
+        # Method 2: Box geometry at origin, then moved via Placement
+        box_placed = Part.makeBox(10, 20, 30)
+        box_placed.Placement = App.Placement(App.Vector(0, 30, 0), App.Rotation())
+
+        # Verify both boxes appear at the same location
+        self.assertAlmostEqual(box_direct.BoundBox.XMin, box_placed.BoundBox.XMin, places=5)
+        self.assertAlmostEqual(box_direct.BoundBox.YMin, box_placed.BoundBox.YMin, places=5)
+        self.assertAlmostEqual(box_direct.BoundBox.ZMin, box_placed.BoundBox.ZMin, places=5)
+
+        # Mirror both across the XZ plane (Y=0)
+        # A point (x, y, z) mirrors to (x, -y, z)
+        # So box at Y=30..50 should mirror to Y=-50..-30
+        mirror_direct = box_direct.mirror(App.Vector(), App.Vector(0, 1, 0))
+        mirror_placed = box_placed.mirror(App.Vector(), App.Vector(0, 1, 0))
+
+        # The mirrored shapes should have identical bounding boxes
+        self.assertAlmostEqual(
+            mirror_direct.BoundBox.XMin,
+            mirror_placed.BoundBox.XMin,
+            places=5,
+            msg="Mirror with Placement produced different XMin",
+        )
+        self.assertAlmostEqual(
+            mirror_direct.BoundBox.YMin,
+            mirror_placed.BoundBox.YMin,
+            places=5,
+            msg="Mirror with Placement produced different YMin",
+        )
+        self.assertAlmostEqual(
+            mirror_direct.BoundBox.ZMin,
+            mirror_placed.BoundBox.ZMin,
+            places=5,
+            msg="Mirror with Placement produced different ZMin",
+        )
+        self.assertAlmostEqual(
+            mirror_direct.BoundBox.XMax,
+            mirror_placed.BoundBox.XMax,
+            places=5,
+            msg="Mirror with Placement produced different XMax",
+        )
+        self.assertAlmostEqual(
+            mirror_direct.BoundBox.YMax,
+            mirror_placed.BoundBox.YMax,
+            places=5,
+            msg="Mirror with Placement produced different YMax",
+        )
+        self.assertAlmostEqual(
+            mirror_direct.BoundBox.ZMax,
+            mirror_placed.BoundBox.ZMax,
+            places=5,
+            msg="Mirror with Placement produced different ZMax",
+        )
+
+        # Verify the expected mirror result: Y=30..50 mirrors to Y=-50..-30
+        self.assertAlmostEqual(mirror_direct.BoundBox.YMin, -50.0, places=5)
+        self.assertAlmostEqual(mirror_direct.BoundBox.YMax, -30.0, places=5)
+
     def testTopoShapeScale(self):
         # Act
         scale = self.doc.Box1.Shape.scaled(2)