diff --git a/CHANGELOG b/CHANGELOG
index 49843b72c..e1615c91d 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,7 +1,8 @@
-# 3.29.1 (2026-XX-XX)
+# 3.30.0 (2026-XX-XX)
 
  * Fix the deprecation about omitting parentheses when calling a macro being reported twice for the same call
  * Add the macro name to the deprecation about omitting parentheses when calling a macro
+ * Deprecate cloning a `Twig\Environment` instance
 
 # 3.29.0 (2026-09-18)
 
diff --git a/doc/deprecated.rst b/doc/deprecated.rst
index 668343c27..3f7e8b4ba 100644
--- a/doc/deprecated.rst
+++ b/doc/deprecated.rst
@@ -475,6 +475,11 @@ Testing Utilities
 Environment
 -----------
 
+* Cloning a ``Twig\Environment`` instance is deprecated as of Twig 3.30 and will
+  throw in Twig 4.0. A clone keeps the templates and runtimes created by the
+  original environment, and those resolve their globals, extensions, and charset
+  against the environment that created them. Build a new environment instead.
+
 * The ``Twig\Environment::mergeGlobals()`` method is deprecated as of Twig 3.14
   and will be removed in Twig 4.0:
 
diff --git a/src/Environment.php b/src/Environment.php
index e3b2e7aaa..260458c71 100644
--- a/src/Environment.php
+++ b/src/Environment.php
@@ -144,6 +144,11 @@ class Environment
         $this->addExtension(new OptimizerExtension($options['optimizations']));
     }
 
+    public function __clone()
+    {
+        trigger_deprecation('twig/twig', '3.30', 'Cloning a "%s" instance is deprecated and will throw in Twig 4.0; build a new environment instead.', self::class);
+    }
+
     /**
      * @internal
      */
diff --git a/tests/EnvironmentTest.php b/tests/EnvironmentTest.php
index d311c2677..cc5d3f296 100644
--- a/tests/EnvironmentTest.php
+++ b/tests/EnvironmentTest.php
@@ -96,6 +96,19 @@ class EnvironmentTest extends TestCase
         $twig->load($foreign->load('index'));
     }
 
+    /**
+     * @group legacy
+     */
+    #[Group('legacy')]
+    public function testCloningIsDeprecated(): void
+    {
+        $this->expectDeprecation('Since twig/twig 3.30: Cloning a "Twig\Environment" instance is deprecated and will throw in Twig 4.0; build a new environment instead.');
+
+        $twig = new Environment(new ArrayLoader());
+
+        $this->assertInstanceOf(Environment::class, clone $twig);
+    }
+
     public function escapingStrategyCallback($name)
     {
         return $name;