diff --git a/lib/client.ts b/lib/client.ts
index 268cfbd085..8dc02c1d02 100644
--- a/lib/client.ts
+++ b/lib/client.ts
@@ -113,10 +113,15 @@ export class Client<
       (
         dynamicNspName:
           | Namespace<ListenEvents, EmitEvents, ServerSideEvents>
-          | false
+          | false,
+        race: boolean = false
       ) => {
         if (dynamicNspName) {
-          debug("dynamic namespace %s was created", dynamicNspName);
+          if (race) {
+            debug("dynamic namespace %s already created", dynamicNspName);
+          } else {
+            debug("dynamic namespace %s was created", dynamicNspName);
+          }
           this.doConnect(name, auth);
         } else {
           debug("creation of namespace %s was denied", name);
diff --git a/lib/index.ts b/lib/index.ts
index 72c87ef677..f38819195f 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -188,7 +188,8 @@ export class Server<
     name: string,
     auth: { [key: string]: any },
     fn: (
-      nsp: Namespace<ListenEvents, EmitEvents, ServerSideEvents> | false
+      nsp: Namespace<ListenEvents, EmitEvents, ServerSideEvents> | false,
+      race?: boolean
     ) => void
   ): void {
     if (this.parentNsps.size === 0) return fn(false);
@@ -203,6 +204,10 @@ export class Server<
       nextFn.value(name, auth, (err, allow) => {
         if (err || !allow) {
           run();
+        } else if (this._nsps.has(name)) {
+          // See #4136. It's possible that in the meantime the namespace has
+          // already been created, so we'll have to handle this properly.
+          fn(this._nsps.get(name) as Namespace, true);
         } else {
           const namespace = this.parentNsps
             .get(nextFn.value)!
diff --git a/test/socket.io.ts b/test/socket.io.ts
index 4777796f89..63e7f1b830 100644
--- a/test/socket.io.ts
+++ b/test/socket.io.ts
@@ -15,6 +15,8 @@ import { io as ioc, Socket as ClientSocket } from "socket.io-client";
 import "./support/util";
 import "./utility-methods";
 
+type callback = (err: Error | null, success: boolean) => void;
+
 // Creates a socket.io client for the given server
 function client(srv, nsp?: string | object, opts?: object): ClientSocket {
   if ("object" == typeof nsp) {
@@ -998,6 +1000,46 @@ describe("socket.io", () => {
           const socket = client(srv, "/dynamic-101");
         });
       });
+
+      it("should handle race conditions with dynamic namespaces (#4136)", (done) => {
+        const srv = createServer();
+        const sio = new Server(srv);
+        const counters = {
+          connected: 0,
+          created: 0,
+          events: 0,
+        };
+        const buffer: callback[] = [];
+        sio.on("new_namespace", (namespace) => {
+          counters.created++;
+        });
+        srv.listen(() => {
+          const handler = () => {
+            if (++counters.events === 2) {
+              expect(counters.created).to.equal(1);
+              done();
+            }
+          };
+
+          sio
+            .of((name, query, next) => {
+              buffer.push(next);
+              if (buffer.length === 2) {
+                buffer.forEach((next) => next(null, true));
+              }
+            })
+            .on("connection", (socket) => {
+              if (++counters.connected === 2) {
+                sio.of("/dynamic-101").emit("message");
+              }
+            });
+
+          let one = client(srv, "/dynamic-101");
+          let two = client(srv, "/dynamic-101");
+          one.on("message", handler);
+          two.on("message", handler);
+        });
+      });
     });
   });