diff --git a/types/types.d.ts b/types/types.d.ts
index 031b050..dced83d 100644
--- a/types/types.d.ts
+++ b/types/types.d.ts
@@ -4,6 +4,11 @@ import { FastifyPluginCallback } from 'fastify';
 import type * as Fastify from 'fastify';
 
 declare module 'fastify' {
+  interface FastifyInstance {
+    decryptSession<Request extends Record<string, any> = FastifyRequest>(sessionId: string, request: Request, cookieOpts: FastifySessionPlugin.CookieOptions, callback: Callback): void;
+    decryptSession<Request extends Record<string, any> = FastifyRequest>(sessionId: string, request: Request, callback: Callback): void;
+  }
+
   interface FastifyRequest {
     /** Allows to access or modify the session data. */
     session: Fastify.Session;
diff --git a/types/types.test-d.ts b/types/types.test-d.ts
index b1ec9f9..3352ecb 100644
--- a/types/types.test-d.ts
+++ b/types/types.test-d.ts
@@ -62,6 +62,12 @@ app.register(plugin, {
 
 expectError(app.register(plugin, {}));
 
+expectError(app.decryptSession<string>('sessionId', {}, () => ({})))
+app.decryptSession<{hello: 'world'}>('sessionId', { hello: 'world' }, () => ({}))
+app.decryptSession<{hello: 'world'}>('sessionId', { hello: 'world' }, { domain: '/' }, () => ({}))
+app.decryptSession('sessionId', {}, () => ({}))
+app.decryptSession('sessionId', {}, { domain: '/' }, () => ({}))
+
 app.route({
   method: 'GET',
   url: '/',