diff --git a/packages/type-utils/tests/isTypeReadonly.test.ts b/packages/type-utils/tests/isTypeReadonly.test.ts
index 5ecfc078f988..24fed73342a7 100644
--- a/packages/type-utils/tests/isTypeReadonly.test.ts
+++ b/packages/type-utils/tests/isTypeReadonly.test.ts
@@ -109,6 +109,32 @@ describe('isTypeReadonly', () => {
           ])('handles non fully readonly sets and maps', runTests);
         });
       });
+
+      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);
+            });
+          });
+        });
+      });
     });
 
     describe('treatMethodsAsReadonly', () => {