diff --git a/src/Smartpay/webhooks.ts b/src/Smartpay/webhooks.ts
index 665cdf8..cae14f7 100644
--- a/src/Smartpay/webhooks.ts
+++ b/src/Smartpay/webhooks.ts
@@ -1,4 +1,4 @@
-import { createHmac } from 'crypto';
+import { createHmac, timingSafeEqual } from 'crypto';
 
 import basex from 'base-x';
 // eslint-disable-next-line import/no-extraneous-dependencies
@@ -187,7 +187,20 @@ const webhooksMixin = <T extends Constructor>(Base: T) => {
         secret,
       });
 
-      return signature === calculatedSignature;
+      const signatureBuffer = Buffer.from(signature, 'utf8');
+      const calculatedSignatureBuffer = Buffer.from(
+        calculatedSignature,
+        'utf8'
+      );
+
+      // Compare in constant time to avoid leaking the expected signature
+      // through timing differences. timingSafeEqual requires equal-length
+      // inputs, so a length mismatch is an early (safe) rejection.
+      if (signatureBuffer.length !== calculatedSignatureBuffer.length) {
+        return false;
+      }
+
+      return timingSafeEqual(signatureBuffer, calculatedSignatureBuffer);
     }
 
     static expressWebhookMiddleware(secret: string | Function) {
diff --git a/test/unit.js b/test/unit.js
index 57ecb82..1e66b72 100644
--- a/test/unit.js
+++ b/test/unit.js
@@ -155,6 +155,41 @@ test('Verify Webhook Signature Verification function', function testWebhookSigna
   t.ok(Smartpay.verifyWebhookSignature({ data, signature, secret }));
 });
 
+test('Reject invalid webhook signature', function testWebhookSignatureReject(t) {
+  t.plan(3);
+
+  // eslint-disable-next-line max-len
+  const data = `1653028612220.{"id":"evt_test_dwPfFKu5iSEKyHR2LFj9Lx","object":"event","createdAt":1653028523052,"test":true,"eventData":{"type":"payment.created","version":"2022-02-18","data":{"id":"payment_test_35LxgmF5KM22XKG38BjpJg","object":"payment","test":true,"createdAt":1653028523020,"updatedAt":1653028523020,"amount":200,"currency":"JPY","order":"order_test_RiYq2rthzRHrkKVGeucSwn","reference":"order_ref_1234567","status":"processed","metadata":{}}}}`;
+  const secret = 'gybcsjixKyBW2d4z6iNPlaYzHUMtawnodwZt3W0q';
+  const validSignature =
+    '68007ada8485ea0ceca7c5e879ae860a50412b7af95ab8e81b32a3e13f3c0832';
+
+  // Wrong signature of the same length must be rejected.
+  const wrongSignature =
+    `${validSignature.slice(0, -1)}0` === validSignature
+      ? `${validSignature.slice(0, -1)}1`
+      : `${validSignature.slice(0, -1)}0`;
+
+  t.notOk(
+    Smartpay.verifyWebhookSignature({ data, signature: wrongSignature, secret })
+  );
+
+  // A signature shorter than the expected one must be rejected (and must not
+  // throw from the constant-time comparison, which requires equal lengths).
+  t.notOk(
+    Smartpay.verifyWebhookSignature({ data, signature: 'deadbeef', secret })
+  );
+
+  // A signature longer than the expected one must be rejected.
+  t.notOk(
+    Smartpay.verifyWebhookSignature({
+      data,
+      signature: `${validSignature}00`,
+      secret,
+    })
+  );
+});
+
 test('Test retry policy', async function testRetryPolicy(t) {
   t.plan(2);