diff --git a/src/Keywords/AdditionalPropertiesKeyword.php b/src/Keywords/AdditionalPropertiesKeyword.php
index b3ac59f..a3530c6 100644
--- a/src/Keywords/AdditionalPropertiesKeyword.php
+++ b/src/Keywords/AdditionalPropertiesKeyword.php
@@ -50,7 +50,10 @@ public function validate(ValidationContext $context, Schema $schema): ?Validatio
             return null;
         }
 
-        $props = $context->getUncheckedProperties();
+        $props = array_diff(
+            $context->getUncheckedProperties(),
+            $this->getSchemaProperties($schema)
+        );
 
         if (!$props) {
             return null;
@@ -80,4 +83,18 @@ public function validate(ValidationContext $context, Schema $schema): ?Validatio
 
         return $error;
     }
-}
\ No newline at end of file
+
+    /**
+     * Get the names of the properties defined in the schema.
+     */
+    protected function getSchemaProperties(Schema $schema): array
+    {
+        $data = $schema->info()->data();
+
+        if (!is_object($data) || !isset($data->properties)) {
+            return [];
+        }
+
+        return array_keys(get_object_vars($data->properties));
+    }
+}