diff --git a/CHANGELOG b/CHANGELOG
index cc000d556..07baad84b 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,6 +1,6 @@
 # 3.26.1 (2026-XX-XX)
 
- * n/a
+ * Deprecate the `Twig\Sandbox\SourcePolicyInterface` interface with no replacement
 
 # 3.26.0 (2026-05-20)
 
diff --git a/doc/deprecated.rst b/doc/deprecated.rst
index eb5b51674..5c86b2ad4 100644
--- a/doc/deprecated.rst
+++ b/doc/deprecated.rst
@@ -304,6 +304,10 @@ Sandbox
   deprecated as of Twig 3.12. You will need to explicitly allow them if needed
   in 4.0.
 
+* The ``Twig\Sandbox\SourcePolicyInterface`` interface is deprecated as of Twig
+  3.26.1 with no replacement. Passing an instance to the
+  ``Twig\Extension\SandboxExtension`` constructor triggers a deprecation.
+
 * Deprecate the ``sandbox`` tag, use the ``sandboxed`` option of the
   ``include`` function instead:
 
diff --git a/src/Extension/SandboxExtension.php b/src/Extension/SandboxExtension.php
index 52f096702..0f8f1b4c6 100644
--- a/src/Extension/SandboxExtension.php
+++ b/src/Extension/SandboxExtension.php
@@ -28,6 +28,10 @@ final class SandboxExtension extends AbstractExtension
 
     public function __construct(SecurityPolicyInterface $policy, $sandboxed = false, ?SourcePolicyInterface $sourcePolicy = null)
     {
+        if (null !== $sourcePolicy) {
+            trigger_deprecation('twig/twig', '3.26.1', 'The "%s" interface is deprecated with no replacement, do not pass an instance to "%s".', SourcePolicyInterface::class, self::class);
+        }
+
         $this->policy = $policy;
         $this->sandboxedGlobally = $sandboxed;
         $this->sourcePolicy = $sourcePolicy;
diff --git a/src/Sandbox/SourcePolicyInterface.php b/src/Sandbox/SourcePolicyInterface.php
index b952f1ea6..2daff120c 100644
--- a/src/Sandbox/SourcePolicyInterface.php
+++ b/src/Sandbox/SourcePolicyInterface.php
@@ -17,6 +17,8 @@ use Twig\Source;
  * Interface for a class that can optionally enable the sandbox mode based on a template's Twig\Source.
  *
  * @author Yaakov Saxon
+ *
+ * @deprecated since Twig 3.26.1 with no replacement
  */
 interface SourcePolicyInterface
 {
diff --git a/tests/Extension/SandboxTest.php b/tests/Extension/SandboxTest.php
index a5c9dad37..c78bfc6c1 100644
--- a/tests/Extension/SandboxTest.php
+++ b/tests/Extension/SandboxTest.php
@@ -932,8 +932,13 @@ EOF
         return $twig;
     }
 
+    /**
+     * @group legacy
+     */
     public function testSandboxSourcePolicyEnableReturningFalse()
     {
+        $this->expectDeprecation('Since twig/twig 3.26.1: The "Twig\Sandbox\SourcePolicyInterface" interface is deprecated with no replacement, do not pass an instance to "Twig\Extension\SandboxExtension".');
+
         $twig = $this->getEnvironment(false, [], self::$templates, [], [], [], [], [], new class implements SourcePolicyInterface {
             public function enableSandbox(Source $source): bool
             {
@@ -943,8 +948,13 @@ EOF
         $this->assertEquals('FOO', $twig->load('1_basic')->render(self::$params));
     }
 
+    /**
+     * @group legacy
+     */
     public function testSandboxSourcePolicyEnableReturningTrue()
     {
+        $this->expectDeprecation('Since twig/twig 3.26.1: The "Twig\Sandbox\SourcePolicyInterface" interface is deprecated with no replacement, do not pass an instance to "Twig\Extension\SandboxExtension".');
+
         $twig = $this->getEnvironment(false, [], self::$templates, [], [], [], [], [], new class implements SourcePolicyInterface {
             public function enableSandbox(Source $source): bool
             {
@@ -955,8 +965,13 @@ EOF
         $twig->load('1_basic')->render([]);
     }
 
+    /**
+     * @group legacy
+     */
     public function testSandboxSourcePolicyFalseDoesntOverrideOtherEnables()
     {
+        $this->expectDeprecation('Since twig/twig 3.26.1: The "Twig\Sandbox\SourcePolicyInterface" interface is deprecated with no replacement, do not pass an instance to "Twig\Extension\SandboxExtension".');
+
         $twig = $this->getEnvironment(true, [], self::$templates, [], [], [], [], [], new class implements SourcePolicyInterface {
             public function enableSandbox(Source $source): bool
             {
@@ -968,10 +983,14 @@ EOF
     }
 
     /**
+     * @group legacy
+     *
      * @dataProvider provideSourcePolicyArrowBlockedTemplates
      */
     public function testSourcePolicyBlocksNonClosureCallableInArrow(string $template)
     {
+        $this->expectDeprecation('Since twig/twig 3.26.1: The "Twig\Sandbox\SourcePolicyInterface" interface is deprecated with no replacement, do not pass an instance to "Twig\Extension\SandboxExtension".');
+
         $sourcePolicy = new class implements SourcePolicyInterface {
             public function enableSandbox(Source $source): bool
             {
@@ -997,8 +1016,13 @@ EOF
         yield 'has every' => ['{{ [1,2] has every "is_int" ? "yes" : "no" }}'];
     }
 
+    /**
+     * @group legacy
+     */
     public function testSourcePolicyAllowsClosureInArrow()
     {
+        $this->expectDeprecation('Since twig/twig 3.26.1: The "Twig\Sandbox\SourcePolicyInterface" interface is deprecated with no replacement, do not pass an instance to "Twig\Extension\SandboxExtension".');
+
         $sourcePolicy = new class implements SourcePolicyInterface {
             public function enableSandbox(Source $source): bool
             {
@@ -1015,6 +1039,7 @@ EOF
      */
     public function testNonSandboxedSourcePolicyAllowsNonClosureCallable()
     {
+        $this->expectDeprecation('Since twig/twig 3.26.1: The "Twig\Sandbox\SourcePolicyInterface" interface is deprecated with no replacement, do not pass an instance to "Twig\Extension\SandboxExtension".');
         $this->expectDeprecation('Since twig/twig 3.15: Passing a callable that is not a PHP \Closure as an argument to the "sort" filter is deprecated.');
 
         $sourcePolicy = new class implements SourcePolicyInterface {
@@ -1048,8 +1073,13 @@ EOF
         $this->assertSame('foo:off', $twig->load('index')->render([]));
     }
 
+    /**
+     * @group legacy
+     */
     public function testNeedsIsSandboxedFilterFollowsSourcePolicy()
     {
+        $this->expectDeprecation('Since twig/twig 3.26.1: The "Twig\Sandbox\SourcePolicyInterface" interface is deprecated with no replacement, do not pass an instance to "Twig\Extension\SandboxExtension".');
+
         $twig = $this->getEnvironment(false, [], [
             'in' => '{{ "foo"|sandbox_aware }}',
             'out' => '{{ "foo"|sandbox_aware }}',