diff --git a/.changeset/inherit-default-hook-on-route.md b/.changeset/inherit-default-hook-on-route.md
new file mode 100644
index 000000000..a06218063
--- /dev/null
+++ b/.changeset/inherit-default-hook-on-route.md
@@ -0,0 +1,5 @@
+---
+'@hono/zod-openapi': patch
+---
+
+fix: inherit `defaultHook` from parent app on nested routes mounted via `app.route()`
diff --git a/packages/zod-openapi/src/index.test.ts b/packages/zod-openapi/src/index.test.ts
index 21d7e2454..fe737d2bb 100644
--- a/packages/zod-openapi/src/index.test.ts
+++ b/packages/zod-openapi/src/index.test.ts
@@ -1083,6 +1083,28 @@ describe('Routers', () => {
     })
     expect(res.status).toBe(200)
   })
+
+  it('Should apply the parent app defaultHook to nested routes mounted via app.route()', async () => {
+    const subApp = new OpenAPIHono().openapi(route, (c) => c.json({ id: 123 }))
+
+    const app = new OpenAPIHono({
+      defaultHook: (result, c) => {
+        if (!result.success) {
+          return c.json({ ok: false, source: 'parentDefaultHook' }, 422)
+        }
+      },
+    }).route('/api', subApp)
+
+    const res = await app.request('/api/posts', {
+      method: 'POST',
+      body: JSON.stringify({ id: 'not-a-number' }),
+      headers: {
+        'Content-Type': 'application/json',
+      },
+    })
+    expect(res.status).toBe(422)
+    expect(await res.json()).toEqual({ ok: false, source: 'parentDefaultHook' })
+  })
 })
 
 describe('Multi params', () => {
diff --git a/packages/zod-openapi/src/index.ts b/packages/zod-openapi/src/index.ts
index cfa40e54f..3fc2a1c87 100644
--- a/packages/zod-openapi/src/index.ts
+++ b/packages/zod-openapi/src/index.ts
@@ -576,7 +576,7 @@ export class OpenAPIHono<
             ? MaybePromise<RouteConfigToTypedResponse<R>> | undefined
             : MaybePromise<RouteConfigToTypedResponse<R>> | MaybePromise<Response> | undefined
         >
-      | undefined = this.defaultHook
+      | undefined = undefined // eslint-disable-line @typescript-eslint/no-useless-default-assignment
   ): OpenAPIHono<
     E,
     S & ToSchema<R['method'], MergePath<BasePath, P>, I, RouteConfigToTypedResponse<R>>,
@@ -586,25 +586,34 @@ export class OpenAPIHono<
       this.openAPIRegistry.registerPath(route)
     }
 
+    // Resolve the defaultHook lazily so nested apps mounted via `route()`
+    // can inherit the parent's defaultHook. See #1306.
+    /* eslint-disable @typescript-eslint/no-unsafe-return, @typescript-eslint/no-unsafe-call */
+    const effectiveHook =
+      hook ??
+      ((result: any, c: any) =>
+        this.defaultHook ? (this.defaultHook as any)(result, c) : undefined)
+    /* eslint-enable @typescript-eslint/no-unsafe-return, @typescript-eslint/no-unsafe-call */
+
     const validators: MiddlewareHandler[] = []
 
     if (route.request?.query) {
-      const validator = zValidator('query', route.request.query as any, hook as any)
+      const validator = zValidator('query', route.request.query as any, effectiveHook as any)
       validators.push(validator as any)
     }
 
     if (route.request?.params) {
-      const validator = zValidator('param', route.request.params as any, hook as any)
+      const validator = zValidator('param', route.request.params as any, effectiveHook as any)
       validators.push(validator as any)
     }
 
     if (route.request?.headers) {
-      const validator = zValidator('header', route.request.headers as any, hook as any)
+      const validator = zValidator('header', route.request.headers as any, effectiveHook as any)
       validators.push(validator as any)
     }
 
     if (route.request?.cookies) {
-      const validator = zValidator('cookie', route.request.cookies as any, hook as any)
+      const validator = zValidator('cookie', route.request.cookies as any, effectiveHook as any)
       validators.push(validator as any)
     }
 
@@ -622,7 +631,7 @@ export class OpenAPIHono<
         if (isJSONContentType(mediaType)) {
           // eslint-disable-next-line @typescript-eslint/ban-ts-comment
           // @ts-ignore we can ignore the type error since Zod Validator's types are not used
-          const validator = zValidator('json', schema, hook) as MiddlewareHandler
+          const validator = zValidator('json', schema, effectiveHook as any) as MiddlewareHandler
           if (route.request?.body?.required) {
             validators.push(validator)
           } else {
@@ -641,7 +650,7 @@ export class OpenAPIHono<
         if (isFormContentType(mediaType)) {
           // eslint-disable-next-line @typescript-eslint/ban-ts-comment
           // @ts-ignore we can ignore the type error since Zod Validator's types are not used
-          const validator = zValidator('form', schema, hook) as MiddlewareHandler
+          const validator = zValidator('form', schema, effectiveHook as any) as MiddlewareHandler
           if (route.request?.body?.required) {
             validators.push(validator)
           } else {
@@ -794,6 +803,10 @@ export class OpenAPIHono<
       return this as any
     }
 
+    if (app.defaultHook === undefined && this.defaultHook !== undefined) {
+      app.defaultHook = this.defaultHook
+    }
+
     app.openAPIRegistry.definitions.forEach((def) => {
       switch (def.type) {
         case 'component':