diff --git a/packages/type-utils/tests/isTypeReadonly.test.ts b/packages/type-utils/tests/isTypeReadonly.test.ts
index 7153b44d7769..7f806c830e0a 100644
--- a/packages/type-utils/tests/isTypeReadonly.test.ts
+++ b/packages/type-utils/tests/isTypeReadonly.test.ts
@@ -215,5 +215,31 @@ describe('isTypeReadonly', () => {
         });
       });
     });
+
+    describe('IndexSignature', () => {
+      describe('is readonly', () => {
+        it('handles readonly PropertySignature inside a readonly IndexSignature', () => {
+          const { type, checker } = getType(
+            `type Test = { readonly [key: string]: { readonly foo: readonly string[]; }; };`,
+          );
+
+          const result = isTypeReadonly(checker, type);
+          expect(result).toBe(true);
+        });
+      });
+
+      describe('is not readonly', () => {
+        describe('default options', () => {
+          it('fails with a mutable PropertySignature inside a readonly IndexSignature', () => {
+            const { type, checker } = getType(
+              `type Test = { readonly [key: string]: { foo: string[]; }; };`,
+            );
+
+            const result = isTypeReadonly(checker, type);
+            expect(result).toBe(false);
+          });
+        });
+      });
+    });
   });
 });