diff --git a/packages/flutter/lib/src/material/bottom_app_bar.dart b/packages/flutter/lib/src/material/bottom_app_bar.dart
index b0fd25ec4d..04827da7f0 100644
--- a/packages/flutter/lib/src/material/bottom_app_bar.dart
+++ b/packages/flutter/lib/src/material/bottom_app_bar.dart
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'bottom_app_bar_theme.dart';
@@ -271,6 +272,7 @@ class BottomAppBar extends StatefulWidget {
 
 class _BottomAppBarState extends State<BottomAppBar> {
   late ValueListenable<ScaffoldGeometry> geometryListenable;
+  final GlobalKey materialKey = GlobalKey();
   static const double _defaultElevation = 8.0;
 
   @override
@@ -285,10 +287,11 @@ class _BottomAppBarState extends State<BottomAppBar> {
     final NotchedShape? notchedShape = widget.shape ?? babTheme.shape;
     final CustomClipper<Path> clipper = notchedShape != null
       ? _BottomAppBarClipper(
-        geometry: geometryListenable,
-        shape: notchedShape,
-        notchMargin: widget.notchMargin,
-      )
+          geometry: geometryListenable,
+          shape: notchedShape,
+          materialKey: materialKey,
+          notchMargin: widget.notchMargin,
+        )
       : const ShapeBorderClipper(shape: RoundedRectangleBorder());
     final double elevation = widget.elevation ?? babTheme.elevation ?? _defaultElevation;
     final Color color = widget.color ?? babTheme.color ?? Theme.of(context).bottomAppBarColor;
@@ -299,6 +302,7 @@ class _BottomAppBarState extends State<BottomAppBar> {
       color: effectiveColor,
       clipBehavior: widget.clipBehavior,
       child: Material(
+        key: materialKey,
         type: MaterialType.transparency,
         child: widget.child == null
           ? null
@@ -312,6 +316,7 @@ class _BottomAppBarClipper extends CustomClipper<Path> {
   const _BottomAppBarClipper({
     required this.geometry,
     required this.shape,
+    required this.materialKey,
     required this.notchMargin,
   }) : assert(geometry != null),
        assert(shape != null),
@@ -320,17 +325,21 @@ class _BottomAppBarClipper extends CustomClipper<Path> {
 
   final ValueListenable<ScaffoldGeometry> geometry;
   final NotchedShape shape;
+  final GlobalKey materialKey;
   final double notchMargin;
 
+  // Returns the top of the BottomAppBar in global coordinates.
+  double get bottomNavigationBarTop {
+    final RenderBox? box = materialKey.currentContext?.findRenderObject() as RenderBox?;
+    return box?.localToGlobal(Offset.zero).dy ?? 0;
+  }
+
   @override
   Path getClip(Size size) {
     // button is the floating action button's bounding rectangle in the
     // coordinate system whose origin is at the appBar's top left corner,
     // or null if there is no floating action button.
-    final Rect? button = geometry.value.floatingActionButtonArea?.translate(
-      0.0,
-      geometry.value.bottomNavigationBarTop! * -1.0,
-    );
+    final Rect? button = geometry.value.floatingActionButtonArea?.translate(0.0, bottomNavigationBarTop * -1.0);
     return shape.getOuterPath(Offset.zero & size, button?.inflate(notchMargin));
   }
 
diff --git a/packages/flutter/test/material/bottom_app_bar_test.dart b/packages/flutter/test/material/bottom_app_bar_test.dart
index 9109b546e1..0d4bd5a412 100644
--- a/packages/flutter/test/material/bottom_app_bar_test.dart
+++ b/packages/flutter/test/material/bottom_app_bar_test.dart
@@ -381,6 +381,40 @@ void main() {
     physicalShape = tester.widget(find.byType(PhysicalShape));
     expect(physicalShape.clipBehavior, Clip.antiAliasWithSaveLayer);
   });
+
+  testWidgets('BottomAppBar with shape when Scaffold.bottomNavigationBar == null', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/80878
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
+          floatingActionButton: FloatingActionButton(
+            backgroundColor: Colors.green,
+            child: const Icon(Icons.home),
+            onPressed: () {},
+          ),
+          body: Stack(
+            children: <Widget>[
+              Container(
+                color: Colors.amber,
+              ),
+              Container(
+                alignment: Alignment.bottomCenter,
+                child: BottomAppBar(
+                  color: Colors.green,
+                  shape: const CircularNotchedRectangle(),
+                  child: Container(height: 50),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.getRect(find.byType(FloatingActionButton)), const Rect.fromLTRB(372, 528, 428, 584));
+    expect(tester.getSize(find.byType(BottomAppBar)), const Size(800, 50));
+  });
 }
 
 // The bottom app bar clip path computation is only available at paint time.