diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5c503eb..4787274 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
   PHP, so it should be named `Integer` for consistency with other types in this
   library. When using it, a class alias prevents static analysis from
   complaining.
+* Mark `Guid\Guid` and `Nonstandard\Uuid` classes as `final`.
 
 ### Deprecated
 
diff --git a/src/Guid/Guid.php b/src/Guid/Guid.php
index ea5c165..632b3ff 100644
--- a/src/Guid/Guid.php
+++ b/src/Guid/Guid.php
@@ -34,7 +34,7 @@ use Ramsey\Uuid\UuidInterface;
  *
  * @psalm-immutable
  */
-class Guid extends Uuid implements UuidInterface
+final class Guid extends Uuid implements UuidInterface
 {
     public function __construct(
         Fields $fields,
diff --git a/src/Nonstandard/Uuid.php b/src/Nonstandard/Uuid.php
index 6a34e90..5a7a333 100644
--- a/src/Nonstandard/Uuid.php
+++ b/src/Nonstandard/Uuid.php
@@ -25,7 +25,7 @@ use Ramsey\Uuid\UuidInterface;
  *
  * @psalm-immutable
  */
-class Uuid extends BaseUuid implements UuidInterface
+final class Uuid extends BaseUuid implements UuidInterface
 {
     public function __construct(
         Fields $fields,