diff --git a/packages/engine.io-client/lib/transport.ts b/packages/engine.io-client/lib/transport.ts
index 9bcd30d90..00c8e33c2 100644
--- a/packages/engine.io-client/lib/transport.ts
+++ b/packages/engine.io-client/lib/transport.ts
@@ -45,6 +45,16 @@ export abstract class Transport extends Emitter<
   public query: Record<string, string>;
   public writable: boolean = false;
 
+  /**
+   * Whether doClose() deferred the actual close until the transport is open.
+   * The polling transport sets this when close() is called mid-handshake, as
+   * the close packet can only be written once the handshake completes (see
+   * Polling#doClose). While set, close() skips its synchronous onClose()
+   * call -- the transport calls onClose() itself once the deferred close
+   * runs.
+   */
+  protected closeDeferred: boolean = false;
+
   protected opts: SocketOptions;
   protected supportsBinary: boolean;
   protected readyState: TransportState;
@@ -100,7 +110,13 @@ export abstract class Transport extends Emitter<
   public close() {
     if (this.readyState === "opening" || this.readyState === "open") {
       this.doClose();
-      this.onClose();
+      // doClose() may defer the close until the transport is open (the
+      // polling transport waits for the handshake to complete so the close
+      // packet can be sent), in which case onClose() runs once the deferred
+      // close completes
+      if (!this.closeDeferred) {
+        this.onClose();
+      }
     }
 
     return this;
diff --git a/packages/engine.io-client/lib/transports/polling.ts b/packages/engine.io-client/lib/transports/polling.ts
index 2f09e8021..32509a8b0 100644
--- a/packages/engine.io-client/lib/transports/polling.ts
+++ b/packages/engine.io-client/lib/transports/polling.ts
@@ -95,6 +95,27 @@ export abstract class Polling extends Transport {
 
       // otherwise bypass onData and handle the message
       this.onPacket(packet);
+
+      if (this.closeDeferred && "open" === packet.type) {
+        // close() was called while the handshake was still in flight and it
+        // just completed, so the deferred close packet can now be written.
+        // The sid is read from the open packet itself, as the Socket may have
+        // already dropped its listeners (see Socket#_onClose).
+        this.closeDeferred = false;
+        if (!this.query.sid && typeof packet.data === "string") {
+          try {
+            const sid = JSON.parse(packet.data).sid;
+            if (sid) {
+              this.query.sid = sid;
+            }
+          } catch (e) {
+            debug("could not parse sid from open packet");
+          }
+        }
+        debug("writing deferred close packet");
+        this.write([{ type: "close" }]);
+        this.onClose();
+      }
     };
 
     // decode payload
@@ -127,12 +148,16 @@ export abstract class Polling extends Transport {
 
     if ("open" === this.readyState) {
       debug("transport open - closing");
+      this.closeDeferred = false;
       close();
     } else {
-      // in case we're trying to close while
-      // handshaking is in progress (GH-164)
+      // in case we're trying to close while handshaking is in progress
+      // (GH-164), wait for the handshake to complete before writing the close
+      // packet. This intentionally does not rely on a one-off "open"
+      // listener: the Socket clears all transport listeners when it closes
+      // (see Socket#_onClose), which would silently drop the deferred close.
       debug("transport not open - deferring close");
-      this.once("open", close);
+      this.closeDeferred = true;
     }
   }
 
diff --git a/packages/engine.io-client/test/transport.js b/packages/engine.io-client/test/transport.js
index d89057c00..8005051f7 100644
--- a/packages/engine.io-client/test/transport.js
+++ b/packages/engine.io-client/test/transport.js
@@ -344,6 +344,86 @@ describe("Transport", () => {
     });
   }
 
+  describe("close during handshake", () => {
+    const OPEN_PACKET =
+      '0{"sid":"close-during-handshake","upgrades":[],"pingInterval":25000,"pingTimeout":20000,"maxPayload":1000000}';
+
+    let originalFetch;
+    let fetchCalls;
+    let releaseHandshake;
+
+    beforeEach(() => {
+      originalFetch = globalThis.fetch;
+      fetchCalls = [];
+      let release;
+      const gate = new Promise((resolve) => {
+        release = resolve;
+      });
+      releaseHandshake = release;
+      const okResponse = (text) => ({
+        ok: true,
+        status: 200,
+        headers: { getSetCookie: () => [] },
+        text: () => Promise.resolve(text),
+      });
+      globalThis.fetch = (url, opts = {}) => {
+        const method = (opts.method || "GET").toUpperCase();
+        fetchCalls.push({ method, url, body: opts.body ?? null });
+        if (method === "GET") {
+          // hang the handshake poll until the test releases it
+          return gate.then(() => okResponse(OPEN_PACKET));
+        }
+        return Promise.resolve(okResponse("ok"));
+      };
+    });
+
+    afterEach(() => {
+      globalThis.fetch = originalFetch;
+    });
+
+    const createSocket = () =>
+      new eio.Socket({ transportImplementations: [eio.Fetch] });
+
+    const closePackets = () =>
+      fetchCalls.filter((call) => call.method === "POST" && call.body === "1");
+
+    it("should send the close packet when the transport is closed mid-handshake", (done) => {
+      const socket = createSocket();
+      setTimeout(() => {
+        socket.transport.close();
+        releaseHandshake();
+        setTimeout(() => {
+          const packets = closePackets();
+          expect(packets.length).to.be(1);
+          expect(packets[0].url).to.contain("sid=close-during-handshake");
+          // the transport must not poll again once closed
+          expect(
+            fetchCalls.filter((call) => call.method === "GET").length,
+          ).to.be(1);
+          socket.close();
+          done();
+        }, 50);
+      }, 20);
+    });
+
+    it("should send the close packet when the socket is closed mid-handshake", (done) => {
+      const socket = createSocket();
+      setTimeout(() => {
+        socket.close();
+        releaseHandshake();
+        setTimeout(() => {
+          const packets = closePackets();
+          expect(packets.length).to.be(1);
+          expect(packets[0].url).to.contain("sid=close-during-handshake");
+          expect(
+            fetchCalls.filter((call) => call.method === "GET").length,
+          ).to.be(1);
+          done();
+        }, 50);
+      }, 20);
+    });
+  });
+
   describe("options", () => {
     it("should accept an `extraHeaders` option for XMLHttpRequest in browser", () => {
       const headers = {