diff --git a/lib/Referrals/UrlDecorator.php b/lib/Referrals/UrlDecorator.php
new file mode 100644
index 0000000000..bc7bdaa804
--- /dev/null
+++ b/lib/Referrals/UrlDecorator.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace MailPoet\Referrals;
+
+use MailPoet\Settings\SettingsController;
+use MailPoet\WP\Functions as WPFunctions;
+
+if (!defined('ABSPATH')) exit;
+
+class UrlDecorator {
+
+  /** @var WPFunctions */
+  private $wp;
+
+  /** @var SettingsController */
+  private $settings;
+
+  function __construct(WPFunctions $wp, SettingsController $settings) {
+    $this->wp = $wp;
+    $this->settings = $settings;
+  }
+
+  function decorate($url) {
+    $referral_id = $this->settings->get(ReferralDetector::REFERRAL_SETTING_NAME, null);
+    if ($referral_id === null) {
+      return $url;
+    }
+    return $this->wp->addQueryArg('ref', $referral_id, $url);
+  }
+}
diff --git a/tests/integration/Referrals/UrlDecoratorTest.php b/tests/integration/Referrals/UrlDecoratorTest.php
new file mode 100644
index 0000000000..34a7c27c4d
--- /dev/null
+++ b/tests/integration/Referrals/UrlDecoratorTest.php
@@ -0,0 +1,37 @@
+<?php
+namespace MailPoet\Test\Router;
+
+use MailPoet\Referrals\ReferralDetector;
+use MailPoet\Referrals\UrlDecorator;
+use MailPoet\Settings\SettingsController;
+use MailPoet\WP\Functions as WPFunctions;
+
+class UrlDecoratorTest extends \MailPoetTest {
+  /** @var SettingsController */
+  private $settings;
+
+  /** @var UrlDecorator */
+  private $url_decorator;
+
+  function _before() {
+    parent::_before();
+    $this->settings = new SettingsController();
+    $this->url_decorator = new UrlDecorator(WPFunctions::get(), $this->settings);
+  }
+
+  function testItDoesntDoAnythingWhenNoReferralId() {
+    $this->settings->set(ReferralDetector::REFERRAL_SETTING_NAME, null);
+    $url = 'http://example.com';
+    expect($this->url_decorator->decorate($url))->equals($url);
+  }
+
+  function testItCorrectlyAddsReferralId() {
+    $this->settings->set(ReferralDetector::REFERRAL_SETTING_NAME, 'abcdefgh');
+    expect($this->url_decorator->decorate('http://example.com/'))
+      ->equals('http://example.com/?ref=abcdefgh');
+    expect($this->url_decorator->decorate('http://example.com/?param=value'))
+      ->equals('http://example.com/?param=value&ref=abcdefgh');
+    expect($this->url_decorator->decorate('http://example.com/?param=value#hash/?param=val'))
+      ->equals('http://example.com/?param=value&ref=abcdefgh#hash/?param=val');
+  }
+}