diff --git a/README.md b/README.md
index 707b18c..76ba5da 100644
--- a/README.md
+++ b/README.md
@@ -776,7 +776,7 @@ final class SomeClass
     public function __construct(
         private int|string $simpleUnion,
         
-        /** @var class-string<SomeInterface>|class-string<AnotherInterface> */
+        /** @var class-string<SomeInterface|AnotherInterface> */
         private string $unionOfClassString,
         
         /** @var array<SomeInterface|AnotherInterface> */
diff --git a/src/Type/Parser/Lexer/Token/ClassStringToken.php b/src/Type/Parser/Lexer/Token/ClassStringToken.php
index 2cc8c13..387000d 100644
--- a/src/Type/Parser/Lexer/Token/ClassStringToken.php
+++ b/src/Type/Parser/Lexer/Token/ClassStringToken.php
@@ -10,6 +10,7 @@ use CuyZ\Valinor\Type\Parser\Exception\Scalar\InvalidClassStringSubType;
 use CuyZ\Valinor\Type\Parser\Lexer\TokenStream;
 use CuyZ\Valinor\Type\Type;
 use CuyZ\Valinor\Type\Types\ClassStringType;
+use CuyZ\Valinor\Type\Types\UnionType;
 use CuyZ\Valinor\Utility\IsSingleton;
 
 /** @internal */
@@ -27,7 +28,7 @@ final class ClassStringToken implements TraversingToken
 
         $type = $stream->read();
 
-        if (! $type instanceof ObjectType) {
+        if (! $type instanceof ObjectType && ! $type instanceof UnionType) {
             throw new InvalidClassStringSubType($type);
         }
 
diff --git a/src/Type/Types/ClassStringType.php b/src/Type/Types/ClassStringType.php
index bbf6c75..182d306 100644
--- a/src/Type/Types/ClassStringType.php
+++ b/src/Type/Types/ClassStringType.php
@@ -9,6 +9,7 @@ use CuyZ\Valinor\Type\StringType;
 use CuyZ\Valinor\Type\Type;
 use CuyZ\Valinor\Type\Types\Exception\CannotCastValue;
 use CuyZ\Valinor\Type\Types\Exception\InvalidClassString;
+use CuyZ\Valinor\Type\Types\Exception\InvalidUnionOfClassString;
 use Stringable;
 
 use function class_exists;
@@ -18,12 +19,24 @@ use function is_string;
 /** @api */
 final class ClassStringType implements StringType
 {
-    private ?ObjectType $subType;
+    /** @var ObjectType|UnionType|null */
+    private ?Type $subType;
 
     private string $signature;
 
-    public function __construct(ObjectType $subType = null)
+    /**
+     * @param ObjectType|UnionType $subType
+     */
+    public function __construct(Type $subType = null)
     {
+        if ($subType instanceof UnionType) {
+            foreach ($subType->types() as $type) {
+                if (! $type instanceof ObjectType) {
+                    throw new InvalidUnionOfClassString($subType);
+                }
+            }
+        }
+
         $this->subType = $subType;
         $this->signature = $this->subType
             ? "class-string<$this->subType>"
@@ -44,7 +57,18 @@ final class ClassStringType implements StringType
             return true;
         }
 
-        return is_a($value, $this->subType->className(), true);
+        if ($this->subType instanceof ObjectType) {
+            return is_a($value, $this->subType->className(), true);
+        }
+
+        foreach ($this->subType->types() as $type) {
+            /** @var ObjectType $type */
+            if (is_a($value, $type->className(), true)) {
+                return true;
+            }
+        }
+
+        return false;
     }
 
     public function matches(Type $other): bool
@@ -89,18 +113,17 @@ final class ClassStringType implements StringType
 
         $value = (string)$value; // @phpstan-ignore-line
 
-        if (! $this->subType) {
-            return $value;
+        if (! $this->accepts($value)) {
+            throw new InvalidClassString($value, $this->subType);
         }
 
-        if (is_a($value, $this->subType->className(), true)) {
-            return $value;
-        }
-
-        throw new InvalidClassString($value, $this->subType);
+        return $value;
     }
 
-    public function subType(): ?ObjectType
+    /**
+     * @return ObjectType|UnionType|null
+     */
+    public function subType(): ?Type
     {
         return $this->subType;
     }
diff --git a/src/Type/Types/Exception/InvalidClassString.php b/src/Type/Types/Exception/InvalidClassString.php
index 59aa5cf..5357795 100644
--- a/src/Type/Types/Exception/InvalidClassString.php
+++ b/src/Type/Types/Exception/InvalidClassString.php
@@ -4,17 +4,38 @@ declare(strict_types=1);
 
 namespace CuyZ\Valinor\Type\Types\Exception;
 
+use CuyZ\Valinor\Type\ObjectType;
 use CuyZ\Valinor\Type\Type;
+use CuyZ\Valinor\Type\Types\UnionType;
 use LogicException;
 
+use function count;
+use function implode;
+
 /** @api */
 final class InvalidClassString extends LogicException implements CastError
 {
-    public function __construct(string $raw, Type $type)
+    /**
+     * @param ObjectType|UnionType|null $type
+     */
+    public function __construct(string $raw, ?Type $type)
     {
-        parent::__construct(
-            "Invalid class string `$raw`, it must be a subtype of `$type`.",
-            1608132562
-        );
+        $types = [];
+
+        if ($type instanceof ObjectType) {
+            $types = [$type];
+        } elseif ($type instanceof UnionType) {
+            $types = $type->types();
+        }
+
+        $message = "Invalid class string `$raw`.";
+
+        if (count($types) > 1) {
+            $message = "Invalid class string `$raw`, it must be one of `" . implode('`, `', $types) . "`.";
+        } elseif (count($types) === 1) {
+            $message = "Invalid class string `$raw`, it must be a subtype of `$type`.";
+        }
+
+        parent::__construct($message, 1608132562);
     }
 }
diff --git a/src/Type/Types/Exception/InvalidUnionOfClassString.php b/src/Type/Types/Exception/InvalidUnionOfClassString.php
new file mode 100644
index 0000000..b7696b9
--- /dev/null
+++ b/src/Type/Types/Exception/InvalidUnionOfClassString.php
@@ -0,0 +1,21 @@
+<?php
+
+declare(strict_types=1);
+
+namespace CuyZ\Valinor\Type\Types\Exception;
+
+use CuyZ\Valinor\Type\Parser\Exception\InvalidType;
+use CuyZ\Valinor\Type\Types\UnionType;
+use LogicException;
+
+/** @internal */
+final class InvalidUnionOfClassString extends LogicException implements InvalidType
+{
+    public function __construct(UnionType $type)
+    {
+        parent::__construct(
+            "Type `$type` contains invalid class string element(s).",
+            1648830951
+        );
+    }
+}
diff --git a/tests/Functional/Type/Parser/Lexer/NativeLexerTest.php b/tests/Functional/Type/Parser/Lexer/NativeLexerTest.php
index 788b304..e8e4638 100644
--- a/tests/Functional/Type/Parser/Lexer/NativeLexerTest.php
+++ b/tests/Functional/Type/Parser/Lexer/NativeLexerTest.php
@@ -425,6 +425,11 @@ final class NativeLexerTest extends TestCase
                 'transformed' => 'class-string<DateTimeInterface>',
                 'type' => ClassStringType::class,
             ],
+            'Class string of union' => [
+                'raw' => 'class-string<DateTimeInterface|stdClass>',
+                'transformed' => 'class-string<DateTimeInterface|stdClass>',
+                'type' => ClassStringType::class,
+            ],
             'Class name' => [
                 'raw' => stdClass::class,
                 'transformed' => stdClass::class,
diff --git a/tests/Unit/Type/Types/ClassStringTypeTest.php b/tests/Unit/Type/Types/ClassStringTypeTest.php
index 4b37399..7f614a5 100644
--- a/tests/Unit/Type/Types/ClassStringTypeTest.php
+++ b/tests/Unit/Type/Types/ClassStringTypeTest.php
@@ -7,6 +7,7 @@ namespace CuyZ\Valinor\Tests\Unit\Type\Types;
 use CuyZ\Valinor\Tests\Fake\Type\FakeObjectType;
 use CuyZ\Valinor\Tests\Fake\Type\FakeType;
 use CuyZ\Valinor\Tests\Fixture\Object\StringableObject;
+use CuyZ\Valinor\Type\Types\Exception\InvalidUnionOfClassString;
 use CuyZ\Valinor\Type\Types\NativeStringType;
 use CuyZ\Valinor\Type\Types\ClassStringType;
 use CuyZ\Valinor\Type\Types\Exception\CannotCastValue;
@@ -22,13 +23,31 @@ use stdClass;
 
 final class ClassStringTypeTest extends TestCase
 {
-    public function test_subtype_can_be_retrieved(): void
+    public function test_string_subtype_can_be_retrieved(): void
     {
         $subType = new FakeObjectType();
 
         self::assertSame($subType, (new ClassStringType($subType))->subType());
     }
 
+    public function test_union_of_string_subtype_can_be_retrieved(): void
+    {
+        $subType = new UnionType(new FakeObjectType(), new FakeObjectType());
+
+        self::assertSame($subType, (new ClassStringType($subType))->subType());
+    }
+
+    public function test_union_with_invalid_type_throws_exception(): void
+    {
+        $type = new UnionType(new FakeObjectType(), new FakeType());
+
+        $this->expectException(InvalidUnionOfClassString::class);
+        $this->expectExceptionCode(1648830951);
+        $this->expectExceptionMessage("Type `$type` contains invalid class string element(s).");
+
+        new ClassStringType($type);
+    }
+
     public function test_accepts_correct_values(): void
     {
         $classStringType = new ClassStringType();
@@ -62,6 +81,26 @@ final class ClassStringTypeTest extends TestCase
         self::assertFalse($classStringType->accepts(stdClass::class));
     }
 
+    public function test_accepts_correct_values_with_union_sub_type(): void
+    {
+        $type = new UnionType(new FakeObjectType(DateTimeInterface::class), new FakeObjectType(stdClass::class));
+        $classStringType = new ClassStringType($type);
+
+        self::assertTrue($classStringType->accepts(DateTime::class));
+        self::assertTrue($classStringType->accepts(DateTimeImmutable::class));
+        self::assertTrue($classStringType->accepts(DateTimeInterface::class));
+
+        self::assertTrue($classStringType->accepts(stdClass::class));
+    }
+
+    public function test_does_not_accept_incorrect_values_with_union_sub_type(): void
+    {
+        $unionType = new UnionType(new FakeObjectType(DateTime::class), new FakeObjectType(stdClass::class));
+        $classStringType = new ClassStringType($unionType);
+
+        self::assertFalse($classStringType->accepts(DateTimeImmutable::class));
+    }
+
     public function test_can_cast_stringable_value(): void
     {
         self::assertTrue((new ClassStringType())->canCast('foo'));
@@ -111,6 +150,17 @@ final class ClassStringTypeTest extends TestCase
     }
 
     public function test_cast_invalid_class_string_throws_exception(): void
+    {
+        $classStringObject = new StringableObject('foo');
+
+        $this->expectException(InvalidClassString::class);
+        $this->expectExceptionCode(1608132562);
+        $this->expectExceptionMessage("Invalid class string `foo`.");
+
+        (new ClassStringType())->cast($classStringObject);
+    }
+
+    public function test_cast_invalid_class_string_of_object_type_throws_exception(): void
     {
         $objectType = new FakeObjectType();
         $classStringObject = new StringableObject(DateTimeInterface::class);
@@ -122,6 +172,18 @@ final class ClassStringTypeTest extends TestCase
         (new ClassStringType($objectType))->cast($classStringObject);
     }
 
+    public function test_cast_invalid_class_string_of_union_type_throws_exception(): void
+    {
+        $unionType = new UnionType(new FakeObjectType(DateTime::class), new FakeObjectType(stdClass::class));
+        $classStringObject = new StringableObject(DateTimeInterface::class);
+
+        $this->expectException(InvalidClassString::class);
+        $this->expectExceptionCode(1608132562);
+        $this->expectExceptionMessage("Invalid class string `DateTimeInterface`, it must be one of `DateTime`, `stdClass`.");
+
+        (new ClassStringType($unionType))->cast($classStringObject);
+    }
+
     public function test_string_value_is_correct(): void
     {
         $objectType = new FakeObjectType();