diff --git a/core/modules/hal/src/Normalizer/ContentEntityNormalizer.php b/core/modules/hal/src/Normalizer/ContentEntityNormalizer.php
index dc05563a21d3d65a9df34d5b806226839e809df7..2ed0a8fb4a930d96ee8340c5a2aec74f6c5f4010 100644
--- a/core/modules/hal/src/Normalizer/ContentEntityNormalizer.php
+++ b/core/modules/hal/src/Normalizer/ContentEntityNormalizer.php
@@ -123,7 +123,7 @@ public function normalize($entity, $format = NULL, array $context = []) {
    *
    * @throws \Symfony\Component\Serializer\Exception\UnexpectedValueException
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     // Get type, necessary for determining which bundle to create.
     if (!isset($data['_links']['type'])) {
       throw new UnexpectedValueException('The type link relation must be specified.');
diff --git a/core/modules/hal/src/Normalizer/FieldItemNormalizer.php b/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
index de7ff4b771b7676db3de8095e50935d8a70fdd9b..f4d8447ee19964c7406383fec980f3bce0f18b15 100644
--- a/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
+++ b/core/modules/hal/src/Normalizer/FieldItemNormalizer.php
@@ -38,7 +38,7 @@ public function normalize($field_item, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     if (!isset($context['target_instance'])) {
       throw new InvalidArgumentException('$context[\'target_instance\'] must be set to denormalize with the FieldItemNormalizer');
     }
diff --git a/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php b/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php
index efdae5241dc26e8351e2088cae68679ed7036259..a5715dedae7daf50377edde7a0f35b2d66fdd4a4 100644
--- a/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php
+++ b/core/modules/jsonapi/src/Normalizer/EntityDenormalizerBase.php
@@ -81,7 +81,7 @@ public function normalize($object, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     if (empty($context['resource_type']) || !$context['resource_type'] instanceof ResourceType) {
       throw new PreconditionFailedHttpException('Missing context during denormalization.');
     }
diff --git a/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php b/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php
index 9409aab03e6088195f312d8df1e98125b96d5391..7a26ebe62816b04246702af295d62b54d2480701 100644
--- a/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php
+++ b/core/modules/jsonapi/src/Normalizer/FieldItemNormalizer.php
@@ -89,7 +89,7 @@ public function normalize($field_item, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     $item_definition = $context['field_definition']->getItemDefinition();
     assert($item_definition instanceof FieldItemDataDefinitionInterface);
 
diff --git a/core/modules/jsonapi/src/Normalizer/FieldNormalizer.php b/core/modules/jsonapi/src/Normalizer/FieldNormalizer.php
index aff9f2ff72389e1b95426e2171b59d9228232459..eaa1d56ecf53561edfe3427c3c301201ba8afb9b 100644
--- a/core/modules/jsonapi/src/Normalizer/FieldNormalizer.php
+++ b/core/modules/jsonapi/src/Normalizer/FieldNormalizer.php
@@ -42,7 +42,7 @@ public function normalize($field, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     $field_definition = $context['field_definition'];
     assert($field_definition instanceof FieldDefinitionInterface);
     $resource_type = $context['resource_type'];
diff --git a/core/modules/jsonapi/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php b/core/modules/jsonapi/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
index 010645d7e6f5bd32d3fe2b019d2728caf57de742..0c9d45fca20f8d7f2acf8e3ca24b555e0fda61b4 100644
--- a/core/modules/jsonapi/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
+++ b/core/modules/jsonapi/src/Normalizer/JsonApiDocumentTopLevelNormalizer.php
@@ -70,7 +70,7 @@ public function __construct(EntityTypeManagerInterface $entity_type_manager, Res
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     $resource_type = $context['resource_type'];
 
     // Validate a few common errors in document formatting.
diff --git a/core/modules/jsonapi/src/Normalizer/ResourceIdentifierNormalizer.php b/core/modules/jsonapi/src/Normalizer/ResourceIdentifierNormalizer.php
index 8e65b7789c723050a7dd5d03b8c711f7714541ae..2b9837ae0d3d84cf4d87771c412faefa48e2337d 100644
--- a/core/modules/jsonapi/src/Normalizer/ResourceIdentifierNormalizer.php
+++ b/core/modules/jsonapi/src/Normalizer/ResourceIdentifierNormalizer.php
@@ -63,7 +63,7 @@ public function normalize($object, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     // If we get here, it's via a relationship POST/PATCH.
     /** @var \Drupal\jsonapi\ResourceType\ResourceType $resource_type */
     $resource_type = $context['resource_type'];
diff --git a/core/modules/jsonapi/src/Serializer/Serializer.php b/core/modules/jsonapi/src/Serializer/Serializer.php
index cb45e8984cc3b43959eac50b414c781689a29763..745fa30465a73f5eed64c4ad62bbaaf952830578 100644
--- a/core/modules/jsonapi/src/Serializer/Serializer.php
+++ b/core/modules/jsonapi/src/Serializer/Serializer.php
@@ -70,7 +70,7 @@ public function normalize($data, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $type, $format = NULL, array $context = []) {
+  public function denormalize($data, $type, $format = NULL, array $context = []): mixed {
     if ($this->selfSupportsDenormalization($data, $type, $format, $context)) {
       return parent::denormalize($data, $type, $format, $context);
     }
diff --git a/core/modules/jsonapi/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php b/core/modules/jsonapi/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php
index 9dd4851574e2715165612c8b4d2ee9d31ec215b1..7ceddc26346cb43b4bc2cc190e09feecf405c4ba 100644
--- a/core/modules/jsonapi/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php
+++ b/core/modules/jsonapi/tests/modules/jsonapi_test_data_type/src/Normalizer/StringNormalizer.php
@@ -26,7 +26,7 @@ public function normalize($object, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     return str_replace('NOT', 'super', $data);
   }
 
diff --git a/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php b/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php
index f7f7ee2e1391ff5c14a10363667dce7f50aef9bd..a5d1e4f4915f6460b6057f80a426b3a856605dc1 100644
--- a/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/ConfigEntityNormalizer.php
@@ -24,7 +24,7 @@ public function normalize($object, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     return parent::denormalize(static::getDataWithoutInternals($data), $class, $format, $context);
   }
 
diff --git a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
index 10d0037bf39e95fa952c2f20e81373288ee52aed..458425369050adcd0975d610844560faf98cb1e4 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeIso8601Normalizer.php
@@ -54,7 +54,7 @@ public function normalize($datetime, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     // @todo Move the date-only handling out of here in https://www.drupal.org/project/drupal/issues/2958416.
     if (isset($context['target_instance'])) {
       $field_definition = $context['target_instance']->getFieldDefinition();
diff --git a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
index 42a715092b75313795e330541d41a74f12d6c917..61cb1a369fa4f1e170a4a85e7efb4d8cc0940c88 100644
--- a/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/DateTimeNormalizer.php
@@ -85,7 +85,7 @@ protected function getNormalizationTimezone() {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     // This only knows how to denormalize datetime strings and timestamps. If
     // something else is received, let validation constraints handle this.
     if (!is_string($data) && !is_numeric($data)) {
diff --git a/core/modules/serialization/src/Normalizer/EntityNormalizer.php b/core/modules/serialization/src/Normalizer/EntityNormalizer.php
index 83ba556e0a77459a27641646f65c7d5e02ae7ce4..d8670fa5b2599b4cb89d632c22edabf0c88bae55 100644
--- a/core/modules/serialization/src/Normalizer/EntityNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/EntityNormalizer.php
@@ -40,7 +40,7 @@ public function __construct(EntityTypeManagerInterface $entity_type_manager, Ent
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     $entity_type_id = $this->determineEntityTypeId($class, $context);
     $entity_type_definition = $this->getEntityTypeDefinition($entity_type_id);
 
diff --git a/core/modules/serialization/src/Normalizer/FieldItemNormalizer.php b/core/modules/serialization/src/Normalizer/FieldItemNormalizer.php
index 2af0afc86ab03111c444ef8453f77dbde567bc7e..820f42b9794286c9c47e3b4fb0a7c3e5540857cf 100644
--- a/core/modules/serialization/src/Normalizer/FieldItemNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/FieldItemNormalizer.php
@@ -22,7 +22,7 @@ class FieldItemNormalizer extends ComplexDataNormalizer implements DenormalizerI
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     if (!isset($context['target_instance'])) {
       throw new InvalidArgumentException('$context[\'target_instance\'] must be set to denormalize with the FieldItemNormalizer');
     }
diff --git a/core/modules/serialization/src/Normalizer/FieldNormalizer.php b/core/modules/serialization/src/Normalizer/FieldNormalizer.php
index 963ac96ef1069bc11a20b3aba9ee15062aacb42f..f012a96857869c8ea0e80dbf0a6fc6334ea2aac3 100644
--- a/core/modules/serialization/src/Normalizer/FieldNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/FieldNormalizer.php
@@ -26,7 +26,7 @@ class FieldNormalizer extends ListNormalizer implements DenormalizerInterface {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     if (!isset($context['target_instance'])) {
       throw new InvalidArgumentException('$context[\'target_instance\'] must be set to denormalize with the FieldNormalizer');
     }
diff --git a/core/modules/serialization/src/Normalizer/TimestampNormalizer.php b/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
index 9be4d6ef949b7152e69c984d3a4e66caf5ff7916..6bb7c03d3172d1692790a6e4e326a519844c2d19 100644
--- a/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
+++ b/core/modules/serialization/src/Normalizer/TimestampNormalizer.php
@@ -39,7 +39,7 @@ protected function getNormalizationTimezone() {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     $denormalized = parent::denormalize($data, $class, $format, $context);
     return $denormalized->getTimestamp();
   }
diff --git a/core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/src/Normalizer/BooleanNormalizer.php b/core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/src/Normalizer/BooleanNormalizer.php
index b1b1bd8cc9ae64ff734bb1549360941d93b98ab4..a2910eab046cc05c8a70b7a05b0fbb2047db1ab8 100644
--- a/core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/src/Normalizer/BooleanNormalizer.php
+++ b/core/modules/serialization/tests/modules/test_datatype_boolean_emoji_normalizer/src/Normalizer/BooleanNormalizer.php
@@ -26,7 +26,7 @@ public function normalize($object, $format = NULL, array $context = []) {
   /**
    * {@inheritdoc}
    */
-  public function denormalize($data, $class, $format = NULL, array $context = []) {
+  public function denormalize($data, $class, $format = NULL, array $context = []): mixed {
     if (!in_array($data, ['👍', '👎'], TRUE)) {
       throw new \UnexpectedValueException('Only 👍 and 👎 are acceptable values.');
     }