diff --git a/CHANGELOG.md b/CHANGELOG.md
index 597124776..2fcd9e7ec 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,10 @@ You can find and compare releases at the [GitHub release page](https://github.co
 
 ## Unreleased
 
+### Added
+
+- Allow injecting pre-built type instances when building a schema from SDL https://github.com/webonyx/graphql-php/issues/681
+
 ## v15.37.2
 
 ### Changed
diff --git a/docs/class-reference.md b/docs/class-reference.md
index 81cd48e81..ad31844ae 100644
--- a/docs/class-reference.md
+++ b/docs/class-reference.md
@@ -2832,6 +2832,7 @@ See [schema definition language docs](schema-definition-language.md) for details
  *
  * @param DocumentNode|Source|string $source
  * @param array<string, bool> $options
+ * @param iterable<Type&NamedType> $types
  *
  * @phpstan-param TypeConfigDecorator|null $typeConfigDecorator
  * @phpstan-param FieldConfigDecorator|null $fieldConfigDecorator
@@ -2849,7 +2850,8 @@ static function build(
     $source,
     ?callable $typeConfigDecorator = null,
     array $options = [],
-    ?callable $fieldConfigDecorator = null
+    ?callable $fieldConfigDecorator = null,
+    iterable $types = []
 ): GraphQL\Type\Schema
 ```
 
@@ -2863,6 +2865,7 @@ static function build(
  * has no resolve methods, so execution will use default resolvers.
  *
  * @param array<string, bool> $options
+ * @param iterable<Type&NamedType> $types
  *
  * @phpstan-param TypeConfigDecorator|null $typeConfigDecorator
  * @phpstan-param FieldConfigDecorator|null $fieldConfigDecorator
@@ -2879,7 +2882,8 @@ static function buildAST(
     GraphQL\Language\AST\DocumentNode $ast,
     ?callable $typeConfigDecorator = null,
     array $options = [],
-    ?callable $fieldConfigDecorator = null
+    ?callable $fieldConfigDecorator = null,
+    iterable $types = []
 ): GraphQL\Type\Schema
 ```
 
diff --git a/docs/schema-definition-language.md b/docs/schema-definition-language.md
index b0caa4efc..3f852d070 100644
--- a/docs/schema-definition-language.md
+++ b/docs/schema-definition-language.md
@@ -55,6 +55,34 @@ $schema = BuildSchema::build($contents, $typeConfigDecorator);
 
 You can learn more about using `$typeConfigDecorator` in [examples/05-type-config-decorator](https://github.com/webonyx/graphql-php/blob/master/examples/05-type-config-decorator).
 
+## Custom scalar and enum types
+
+When building a schema from SDL, scalar types are stubs — they serialize and parse values as-is.
+To attach real behavior (validation, coercion, PHP-backed enum values), pass pre-built type instances via the `types` parameter:
+
+```php
+use GraphQL\Type\Definition\CustomScalarType;
+use GraphQL\Utils\BuildSchema;
+
+$dateType = new CustomScalarType([
+    'name' => 'Date',
+    'serialize' => static fn ($value) => $value->format('Y-m-d'),
+    'parseValue' => static fn ($value) => new DateTimeImmutable($value),
+]);
+
+$schema = BuildSchema::build(
+    file_get_contents('schema.graphql'),
+    null,
+    [],
+    null,
+    [$dateType],
+);
+```
+
+Types whose names match SDL definitions replace them entirely, including their extensions.
+This works for any kind of type, so SDL can reference code-first types by declaring a stub such as `type User`.
+Types whose names are absent from the SDL are registered as extras and remain reachable via `$schema->getType()`.
+
 ## Performance considerations
 
 Method **BuildSchema::build()** produces a [lazy schema](schema-definition.md#lazy-loading-of-types) automatically,
diff --git a/src/Utils/ASTDefinitionBuilder.php b/src/Utils/ASTDefinitionBuilder.php
index 30fd51a81..e7609a3ae 100644
--- a/src/Utils/ASTDefinitionBuilder.php
+++ b/src/Utils/ASTDefinitionBuilder.php
@@ -87,9 +87,13 @@ class ASTDefinitionBuilder
     /** @var array<string, array<int, Node&TypeExtensionNode>> */
     private array $typeExtensionsMap;
 
+    /** @var array<string, Type&NamedType> */
+    private array $typeOverrides;
+
     /**
      * @param array<string, Node&TypeDefinitionNode> $typeDefinitionsMap
      * @param array<string, array<int, Node&TypeExtensionNode>> $typeExtensionsMap
+     * @param array<string, Type&NamedType> $typeOverrides
      *
      * @phpstan-param ResolveType $resolveType
      * @phpstan-param TypeConfigDecorator|null $typeConfigDecorator
@@ -101,13 +105,15 @@ public function __construct(
         array $typeExtensionsMap,
         callable $resolveType,
         ?callable $typeConfigDecorator = null,
-        ?callable $fieldConfigDecorator = null
+        ?callable $fieldConfigDecorator = null,
+        array $typeOverrides = []
     ) {
         $this->typeDefinitionsMap = $typeDefinitionsMap;
         $this->typeExtensionsMap = $typeExtensionsMap;
         $this->resolveType = $resolveType;
         $this->typeConfigDecorator = $typeConfigDecorator;
         $this->fieldConfigDecorator = $fieldConfigDecorator;
+        $this->typeOverrides = $typeOverrides;
 
         $this->cache = Type::builtInTypes();
     }
@@ -264,6 +270,10 @@ private function internalBuildType(string $typeName, ?Node $typeNode = null): Ty
             return $this->cache[$typeName];
         }
 
+        if (isset($this->typeOverrides[$typeName])) {
+            return $this->cache[$typeName] = $this->typeOverrides[$typeName];
+        }
+
         if (isset($this->typeDefinitionsMap[$typeName])) {
             $type = $this->makeSchemaDef($this->typeDefinitionsMap[$typeName]);
 
diff --git a/src/Utils/BuildSchema.php b/src/Utils/BuildSchema.php
index 863946f92..e4ded7744 100644
--- a/src/Utils/BuildSchema.php
+++ b/src/Utils/BuildSchema.php
@@ -14,6 +14,7 @@
 use GraphQL\Language\Parser;
 use GraphQL\Language\Source;
 use GraphQL\Type\Definition\Directive;
+use GraphQL\Type\Definition\NamedType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Schema;
 use GraphQL\Type\SchemaConfig;
@@ -68,8 +69,12 @@ class BuildSchema
      */
     private array $options;
 
+    /** @var iterable<Type&NamedType> */
+    private iterable $types;
+
     /**
      * @param array<string, bool> $options
+     * @param iterable<Type&NamedType> $types
      *
      * @phpstan-param TypeConfigDecorator|null $typeConfigDecorator
      * @phpstan-param BuildSchemaOptions $options
@@ -78,12 +83,14 @@ public function __construct(
         DocumentNode $ast,
         ?callable $typeConfigDecorator = null,
         array $options = [],
-        ?callable $fieldConfigDecorator = null
+        ?callable $fieldConfigDecorator = null,
+        iterable $types = []
     ) {
         $this->ast = $ast;
         $this->typeConfigDecorator = $typeConfigDecorator;
         $this->options = $options;
         $this->fieldConfigDecorator = $fieldConfigDecorator;
+        $this->types = $types;
     }
 
     /**
@@ -92,6 +99,7 @@ public function __construct(
      *
      * @param DocumentNode|Source|string $source
      * @param array<string, bool> $options
+     * @param iterable<Type&NamedType> $types
      *
      * @phpstan-param TypeConfigDecorator|null $typeConfigDecorator
      * @phpstan-param FieldConfigDecorator|null $fieldConfigDecorator
@@ -109,13 +117,14 @@ public static function build(
         $source,
         ?callable $typeConfigDecorator = null,
         array $options = [],
-        ?callable $fieldConfigDecorator = null
+        ?callable $fieldConfigDecorator = null,
+        iterable $types = []
     ): Schema {
         $doc = $source instanceof DocumentNode
             ? $source
             : Parser::parse($source);
 
-        return self::buildAST($doc, $typeConfigDecorator, $options, $fieldConfigDecorator);
+        return self::buildAST($doc, $typeConfigDecorator, $options, $fieldConfigDecorator, $types);
     }
 
     /**
@@ -127,6 +136,7 @@ public static function build(
      * has no resolve methods, so execution will use default resolvers.
      *
      * @param array<string, bool> $options
+     * @param iterable<Type&NamedType> $types
      *
      * @phpstan-param TypeConfigDecorator|null $typeConfigDecorator
      * @phpstan-param FieldConfigDecorator|null $fieldConfigDecorator
@@ -143,9 +153,10 @@ public static function buildAST(
         DocumentNode $ast,
         ?callable $typeConfigDecorator = null,
         array $options = [],
-        ?callable $fieldConfigDecorator = null
+        ?callable $fieldConfigDecorator = null,
+        iterable $types = []
     ): Schema {
-        return (new self($ast, $typeConfigDecorator, $options, $fieldConfigDecorator))->buildSchema();
+        return (new self($ast, $typeConfigDecorator, $options, $fieldConfigDecorator, $types))->buildSchema();
     }
 
     /**
@@ -201,6 +212,27 @@ public function buildSchema(): Schema
                 'subscription' => 'Subscription',
             ];
 
+        /** @var array<string, Type&NamedType> $typeOverrides */
+        $typeOverrides = [];
+        /** @var array<string, Type&NamedType> $extraTypesMap */
+        $extraTypesMap = [];
+        foreach ($this->types as $type) {
+            // @phpstan-ignore function.alreadyNarrowedType, instanceof.alwaysTrue (unnecessary according to types, but can happen during runtime)
+            assert($type instanceof NamedType, 'Types passed to BuildSchema must be named types, got: ' . Utils::printSafe($type) . '.');
+            $typeName = $type->name;
+            $knownType = $typeOverrides[$typeName] ?? $extraTypesMap[$typeName] ?? $type;
+            assert(
+                $knownType === $type,
+                "Schema must contain unique named types but contains multiple types named \"{$typeName}\" (see https://webonyx.github.io/graphql-php/type-definitions/#type-registry).",
+            );
+
+            if (isset($typeDefinitionsMap[$typeName])) {
+                $typeOverrides[$typeName] = $type;
+            } else {
+                $extraTypesMap[$typeName] = $type;
+            }
+        }
+
         $definitionBuilder = new ASTDefinitionBuilder(
             $typeDefinitionsMap,
             $typeExtensionsMap,
@@ -208,7 +240,8 @@ static function (string $typeName): Type {
                 throw self::unknownType($typeName);
             },
             $this->typeConfigDecorator,
-            $this->fieldConfigDecorator
+            $this->fieldConfigDecorator,
+            $typeOverrides
         );
 
         $directives = array_map(
@@ -256,12 +289,16 @@ static function (string $typeName): Type {
                 ->setSubscription(isset($operationTypes['subscription'])
                     ? $definitionBuilder->maybeBuildType($operationTypes['subscription'])
                     : null)
-                ->setTypeLoader(static fn (string $name): ?Type => $definitionBuilder->maybeBuildType($name))
+                ->setTypeLoader(static fn (string $name): ?Type => $definitionBuilder->maybeBuildType($name)
+                    ?? ($extraTypesMap[$name] ?? null))
                 ->setDirectives($directives)
                 ->setAstNode($schemaDef)
-                ->setTypes(fn (): array => array_map(
-                    static fn (TypeDefinitionNode $def): Type => $definitionBuilder->buildType($def->getName()->value),
-                    $typeDefinitionsMap,
+                ->setTypes(fn (): array => array_merge(
+                    array_map(
+                        static fn (TypeDefinitionNode $def): Type => $definitionBuilder->buildType($def->getName()->value),
+                        $typeDefinitionsMap,
+                    ),
+                    array_values($extraTypesMap),
                 ))
         );
     }
diff --git a/tests/Utils/BuildSchemaTest.php b/tests/Utils/BuildSchemaTest.php
index 8cf75f0d4..b79133da3 100644
--- a/tests/Utils/BuildSchemaTest.php
+++ b/tests/Utils/BuildSchemaTest.php
@@ -24,6 +24,7 @@
 use GraphQL\Language\Parser;
 use GraphQL\Language\Printer;
 use GraphQL\Tests\TestCaseBase;
+use GraphQL\Type\Definition\CustomScalarType;
 use GraphQL\Type\Definition\Directive;
 use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\EnumValueDefinition;
@@ -1496,6 +1497,129 @@ interface Hello {
         self::assertSame('My description of Hello', $hello->description);
     }
 
+    public function testBuildSchemaWithTypeOverrides(): void
+    {
+        $sdl = '
+            schema {
+              query: Query
+            }
+
+            type Query {
+              value: MyScalar
+              status: Status
+            }
+
+            scalar MyScalar
+
+            enum Status {
+              ACTIVE
+              INACTIVE
+            }
+        ';
+
+        $myScalar = new CustomScalarType([
+            'name' => 'MyScalar',
+            'serialize' => static fn ($value) => 'serialized:' . $value,
+            'parseValue' => static fn ($value) => 'parsed:' . $value,
+        ]);
+
+        $myEnum = new EnumType([
+            'name' => 'Status',
+            'values' => [
+                'ACTIVE' => [
+                    'value' => 1,
+                ],
+                'INACTIVE' => [
+                    'value' => 0,
+                ],
+            ],
+        ]);
+
+        $extraType = new ObjectType([
+            'name' => 'ExtraType',
+            'fields' => [
+                'id' => \GraphQL\Type\Definition\Type::string(),
+            ],
+        ]);
+
+        $schema = BuildSchema::build($sdl, null, [], null, [$myScalar, $myEnum, $extraType]);
+
+        $scalar = $schema->getType('MyScalar');
+        self::assertSame($myScalar, $scalar);
+
+        $enum = $schema->getType('Status');
+        self::assertSame($myEnum, $enum);
+
+        $extra = $schema->getType('ExtraType');
+        self::assertSame($extraType, $extra);
+
+        // Verify the custom scalar serialize/parseValue are actually used
+        $result = GraphQL::executeQuery(
+            $schema,
+            '{ value }',
+            ['value' => 'hello']
+        );
+        self::assertSame(['value' => 'serialized:hello'], $result->data);
+    }
+
+    public function testBuildSchemaWithObjectTypeOverride(): void
+    {
+        $sdl = '
+            type Query {
+              user: User
+            }
+
+            type User
+        ';
+
+        $user = new ObjectType([
+            'name' => 'User',
+            'fields' => [
+                'name' => [
+                    'type' => Type::string(),
+                    'resolve' => static fn (): string => 'resolved in PHP',
+                ],
+            ],
+        ]);
+
+        $schema = BuildSchema::build($sdl, null, [], null, [$user]);
+        $schema->assertValid();
+
+        $result = GraphQL::executeQuery(
+            $schema,
+            '{ user { name } }',
+            ['user' => []]
+        );
+        self::assertSame(['user' => ['name' => 'resolved in PHP']], $result->data);
+    }
+
+    public function testBuildSchemaAllowsSameTypeInstanceTwice(): void
+    {
+        $date = new CustomScalarType(['name' => 'Date']);
+
+        $schema = BuildSchema::build('type Query { date: Date } scalar Date', null, [], null, [$date, $date]);
+
+        self::assertSame($date, $schema->getType('Date'));
+    }
+
+    public function testBuildSchemaAssertsNamedTypesAtDevelopmentTime(): void
+    {
+        $this->expectException(\AssertionError::class);
+        // @phpstan-ignore-next-line intentionally wrong
+        BuildSchema::build('type Query { date: Date } scalar Date', null, [], null, [
+            Type::nonNull(new CustomScalarType(['name' => 'Date'])),
+        ]);
+    }
+
+    public function testBuildSchemaAssertsUniqueTypeNamesAtDevelopmentTime(): void
+    {
+        $this->expectException(\AssertionError::class);
+        BuildSchema::build('type Query { id: ID }', null, [], null, [
+            new CustomScalarType(['name' => 'Date']),
+            new CustomScalarType(['name' => 'Date']),
+        ]);
+    }
+
     public function testCreatesTypesLazily(): void
     {
         $sdl = '