diff --git a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php
index 02021c8bc5..0b56476c69 100644
--- a/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php
+++ b/src/Symfony/Component/DependencyInjection/Compiler/ResolveInstanceofConditionalsPass.php
@@ -113,14 +113,17 @@ class ResolveInstanceofConditionalsPass implements CompilerPassInterface
                 $definition->setShared($shared);
             }
 
-            $i = \count($instanceofTags);
-            while (0 <= --$i) {
-                foreach ($instanceofTags[$i] as $k => $v) {
-                    foreach ($v as $v) {
-                        if ($definition->hasTag($k) && \in_array($v, $definition->getTag($k))) {
-                            continue;
+            // Don't add tags to service decorators
+            if (null === $definition->getDecoratedService()) {
+                $i = \count($instanceofTags);
+                while (0 <= --$i) {
+                    foreach ($instanceofTags[$i] as $k => $v) {
+                        foreach ($v as $v) {
+                            if ($definition->hasTag($k) && \in_array($v, $definition->getTag($k))) {
+                                continue;
+                            }
+                            $definition->addTag($k, $v);
                         }
-                        $definition->addTag($k, $v);
                     }
                 }
             }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveInstanceofConditionalsPassTest.php b/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveInstanceofConditionalsPassTest.php
index 88cc88b9dc..7fee5194f0 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveInstanceofConditionalsPassTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Compiler/ResolveInstanceofConditionalsPassTest.php
@@ -307,4 +307,26 @@ class ResolveInstanceofConditionalsPassTest extends TestCase
         $this->assertInstanceOf(BoundArgument::class, $bindings['$toto']);
         $this->assertSame(123, $bindings['$toto']->getValues()[0]);
     }
+
+    public function testDecoratorsAreNotAutomaticallyTagged()
+    {
+        $container = new ContainerBuilder();
+
+        $decorator = $container->register('decorator', self::class);
+        $decorator->setDecoratedService('decorated');
+        $decorator->setInstanceofConditionals([
+            parent::class => (new ChildDefinition(''))->addTag('tag'),
+        ]);
+        $decorator->setAutoconfigured(true);
+        $decorator->addTag('manual');
+
+        $container->registerForAutoconfiguration(parent::class)
+            ->addTag('tag')
+        ;
+
+        (new ResolveInstanceofConditionalsPass())->process($container);
+        (new ResolveChildDefinitionsPass())->process($container);
+
+        $this->assertSame(['manual' => [[]]], $container->getDefinition('decorator')->getTags());
+    }
 }