diff --git a/index.d.ts b/index.d.ts
index fc14c73..b59a08f 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -71,6 +71,8 @@ declare namespace Router {
     deriveConstraint<Context>(req: Req<V>, ctx?: Context) : T,
   }
 
+  type QuerystringParser = (s: string) => unknown;
+
   interface Config<V extends HTTPVersion> {
     ignoreTrailingSlash?: boolean;
 
@@ -82,6 +84,8 @@ declare namespace Router {
 
     maxParamLength?: number;
 
+    querystringParser?: QuerystringParser;
+
     defaultRoute?(
       req: Req<V>,
       res: Res<V>