diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 6645863bfd7..0a95132681a 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -4,6 +4,7 @@ Yii Framework 2 Change Log
 2.0.50 under development
 ------------------------
 
+- Bug #17191: Fixed `BaseUrl::isRelative($url)` method in `yii\helpers\BaseUrl` (ggh2e3)
 - Bug #18469: Fixed `Link::serialize(array $links)` method in `yii\web\Link` (ggh2e3)
 - Bug #20040: Fix type `boolean` in `MSSQL` (terabytesoftw)
 - Bug #20005: Fix `yii\console\controllers\ServeController` to specify the router script (terabytesoftw)
diff --git a/framework/helpers/BaseUrl.php b/framework/helpers/BaseUrl.php
index a671123c7cd..dcbab192730 100644
--- a/framework/helpers/BaseUrl.php
+++ b/framework/helpers/BaseUrl.php
@@ -378,7 +378,8 @@ public static function home($scheme = false)
      */
     public static function isRelative($url)
     {
-        return strncmp($url, '//', 2) && strpos($url, '://') === false;
+        $urlComponents = parse_url($url, PHP_URL_SCHEME);
+        return strncmp($url, '//', 2) && empty($urlComponents);
     }
 
     /**
diff --git a/tests/framework/helpers/BaseUrlTest.php b/tests/framework/helpers/BaseUrlTest.php
new file mode 100644
index 00000000000..9bcd39ef981
--- /dev/null
+++ b/tests/framework/helpers/BaseUrlTest.php
@@ -0,0 +1,116 @@
+<?php
+
+namespace yiiunit\framework\helpers;
+
+use PHPUnit\Framework\TestCase;
+use yii\helpers\BaseUrl;
+
+/**
+ * @group helpers
+ */
+class BaseUrlTest extends TestCase
+{
+    /** @dataProvider relativeTrueUrlProvider */
+    public function testIsRelativeWillReturnTrue($url)
+    {
+        $this->assertTrue(BaseUrl::isRelative($url));
+    }
+
+    /** @dataProvider relativeFalseUrlProvider */
+    public function testIsRelativeWillReturnFalse($url)
+    {
+        $this->assertFalse(BaseUrl::isRelative($url));
+    }
+
+    /** @dataProvider ensureSchemeUrlProvider */
+    public function testEnsureScheme($url, $scheme, $expected)
+    {
+        $this->assertEquals($expected, BaseUrl::ensureScheme($url, $scheme));
+    }
+
+    public function ensureSchemeUrlProvider()
+    {
+        return [
+            'relative url and https scheme will return input url' => [
+                'url' => 'acme.com?name=bugs.bunny',
+                'scheme' => 'https',
+                'expected result' => 'acme.com?name=bugs.bunny',
+            ],
+            'relative url and another url as parameter will return input url' => [
+                'url' => 'acme.com/test?tnt-link=https://tnt.com/',
+                'scheme' => 'https',
+                'expected' => 'acme.com/test?tnt-link=https://tnt.com/',
+            ],
+            'url with scheme not a string will return input url' => [
+                'url' => 'acme.com?name=bugs.bunny',
+                'scheme' => 123,
+                'expected' => 'acme.com?name=bugs.bunny',
+            ],
+            'protocol relative url and https scheme will be processed' => [
+                'url' => '//acme.com?characters/list',
+                'scheme' => 'https',
+                'expected' => 'https://acme.com?characters/list',
+            ],
+            'protocol relative url and empty scheme will be returned' => [
+                'url' => '//acme.com?characters/list',
+                'scheme' => '',
+                'expected' => '//acme.com?characters/list',
+            ],
+            'absolute url and empty scheme will create protocol relative url' => [
+                'url' => 'https://acme.com?characters/list',
+                'scheme' => '',
+                'expected' => '//acme.com?characters/list',
+            ],
+            'absolute url and different scheme will be processed' => [
+                'url' => 'http://acme.com/test?tnt-link=https://tnt.com/',
+                'scheme' => 'https',
+                'expected' => 'https://acme.com/test?tnt-link=https://tnt.com/',
+            ]
+        ];
+    }
+
+    public function relativeTrueUrlProvider()
+    {
+        return [
+            'url url without protocol' => [
+                'url' => 'acme.com/tnt-room=123',
+            ],
+            'url without protocol and another url in a parameter value' => [
+                'url' => 'acme.com?tnt-room-link=https://tnt.com',
+            ],
+            'path only' => [
+                'url' => '/path',
+            ],
+            'path with param' => [
+                'url' => '/path=/home/user',
+            ],
+        ];
+    }
+
+    public function relativeFalseUrlProvider()
+    {
+        return [
+            'url with https protocol' => [
+                'url' => 'https://acme.com',
+            ],
+            'url with https protocol and ending slash' => [
+                'url' => 'https://acme.com/',
+            ],
+            'url with https protocol and another url as param value' => [
+                'url' => 'https://acme.com?tnt-link=https://tnt.com',
+            ],
+            'url starting with two slashes' => [
+                'url' => '//example.com',
+            ],
+            'url with ftp protocol' => [
+                'url' => 'ftp://ftp.acme.com/tnt-suppliers.txt',
+            ],
+            'url with http protocol' => [
+                'url' => 'http://no-protection.acme.com',
+            ],
+            'file url' => [
+                'url' => 'file:///home/User/2ndFile.html',
+            ]
+        ];
+    }
+}