diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index f521c80e76..3a03a16965 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -21,6 +21,7 @@ Yii Framework 2 Change Log
 - Bug #18325: Fix `yii\db\pgsql\Schema` to respect non-default PgSQL schema name for data types (theonedemon, silverfire)
 - Bug #18593: Fix setting the `maxlength` attribute for `Html::activeInput()` and `Html::activeTextArea()` based on `length` parameter of validator (BSCheshir)
 - Bug #18592: Fix `yii\db\Command::getRawSql()` to not replace query params in invalid places (sartor)
+- Bug #18590: Fix `yii\web\UrlManager` to instantiate cache only when it's actually needed (bizley)
 
 2.0.41.1 March 04, 2021
 -----------------------
diff --git a/framework/web/UrlManager.php b/framework/web/UrlManager.php
index 13e15e4d20..92c73d3fe9 100644
--- a/framework/web/UrlManager.php
+++ b/framework/web/UrlManager.php
@@ -119,7 +119,7 @@ class UrlManager extends Component
      */
     public $routeParam = 'r';
     /**
-     * @var CacheInterface|array|string the cache object or the application component ID of the cache object.
+     * @var CacheInterface|array|string|bool the cache object or the application component ID of the cache object.
      * This can also be an array that is used to create a [[CacheInterface]] instance in case you do not want to use
      * an application component.
      * Compiled URL rules will be cached through this cache object, if it is available.
@@ -185,17 +185,10 @@ class UrlManager extends Component
         if (!$this->enablePrettyUrl) {
             return;
         }
-        if ($this->cache !== false && $this->cache !== null) {
-            try {
-                $this->cache = Instance::ensure($this->cache, 'yii\caching\CacheInterface');
-            } catch (InvalidConfigException $e) {
-                Yii::warning('Unable to use cache for URL manager: ' . $e->getMessage());
-            }
+
+        if (!empty($this->rules)) {
+            $this->rules = $this->buildRules($this->rules);
         }
-        if (empty($this->rules)) {
-            return;
-        }
-        $this->rules = $this->buildRules($this->rules);
     }
 
     /**
@@ -263,6 +256,23 @@ class UrlManager extends Component
         return $builtRules;
     }
 
+    /**
+     * @return CacheInterface|null|bool
+     */
+    private function ensureCache()
+    {
+        if (!$this->cache instanceof CacheInterface && $this->cache !== false && $this->cache !== null) {
+            try {
+                $this->cache = Instance::ensure($this->cache, 'yii\caching\CacheInterface');
+            } catch (InvalidConfigException $e) {
+                Yii::warning('Unable to use cache for URL manager: ' . $e->getMessage());
+                $this->cache = null;
+            }
+        }
+
+        return $this->cache;
+    }
+
     /**
      * Stores $builtRules to cache, using $rulesDeclaration as a part of cache key.
      *
@@ -274,11 +284,12 @@ class UrlManager extends Component
      */
     protected function setBuiltRulesCache($ruleDeclarations, $builtRules)
     {
-        if (!$this->cache instanceof CacheInterface) {
+        $cache = $this->ensureCache();
+        if (!$cache) {
             return false;
         }
 
-        return $this->cache->set([$this->cacheKey, $this->ruleConfig, $ruleDeclarations], $builtRules);
+        return $cache->set([$this->cacheKey, $this->ruleConfig, $ruleDeclarations], $builtRules);
     }
 
     /**
@@ -292,11 +303,12 @@ class UrlManager extends Component
      */
     protected function getBuiltRulesFromCache($ruleDeclarations)
     {
-        if (!$this->cache instanceof CacheInterface) {
+        $cache = $this->ensureCache();
+        if (!$cache) {
             return false;
         }
 
-        return $this->cache->get([$this->cacheKey, $this->ruleConfig, $ruleDeclarations]);
+        return $cache->get([$this->cacheKey, $this->ruleConfig, $ruleDeclarations]);
     }
 
     /**
diff --git a/tests/framework/web/UrlManagerCreateUrlTest.php b/tests/framework/web/UrlManagerCreateUrlTest.php
index ae95c2a855..a2f894c24b 100644
--- a/tests/framework/web/UrlManagerCreateUrlTest.php
+++ b/tests/framework/web/UrlManagerCreateUrlTest.php
@@ -837,4 +837,25 @@ class UrlManagerCreateUrlTest extends TestCase
         $this->assertInstanceOf(UrlRule::className(), $urlManager->rules[0]);
         $this->assertInstanceOf(CachedUrlRule::className(), $cachedUrlManager->rules[0]);
     }
+
+    public function testNotEnsuringCacheForEmptyRuleset()
+    {
+        $this->mockWebApplication([
+            'components' => [
+                'cache' => ArrayCache::className(),
+            ],
+        ]);
+        // no rules - don't ensure cache
+        $urlManager = $this->getUrlManager([
+            'cache' => 'cache',
+            'rules' => [],
+        ]);
+        $this->assertSame('cache', $urlManager->cache);
+        // with rules - ensure cache
+        $urlManager = $this->getUrlManager([
+            'cache' => 'cache',
+            'rules' => ['/' => 'site/index'],
+        ]);
+        $this->assertInstanceOf(ArrayCache::className(), $urlManager->cache);
+    }
 }