diff --git a/src/Type/PHPUnit/Assert/AssertTypeSpecifyingExtensionHelper.php b/src/Type/PHPUnit/Assert/AssertTypeSpecifyingExtensionHelper.php
index 2276b51..3ea36eb 100644
--- a/src/Type/PHPUnit/Assert/AssertTypeSpecifyingExtensionHelper.php
+++ b/src/Type/PHPUnit/Assert/AssertTypeSpecifyingExtensionHelper.php
@@ -13,7 +13,9 @@
 use PhpParser\Node\Expr\FuncCall;
 use PhpParser\Node\Expr\Instanceof_;
 use PhpParser\Node\Name;
+use PhpParser\Node\Param;
 use PhpParser\Node\Scalar\LNumber;
+use PhpParser\Node\Stmt;
 use PHPStan\Analyser\Scope;
 use PHPStan\Analyser\SpecifiedTypes;
 use PHPStan\Analyser\TypeSpecifier;
@@ -274,6 +276,28 @@ private static function getExpressionResolvers(): array
 				'ObjectHasAttribute' => static function (Scope $scope, Arg $property, Arg $object): FuncCall {
 					return new FuncCall(new Name('property_exists'), [$object, $property]);
 				},
+				'ContainsOnlyInstancesOf' => static function (Scope $scope, Arg $className, Arg $haystack): Expr {
+					return new Expr\BinaryOp\BooleanOr(
+						new Expr\Instanceof_($haystack->value, new Name('Traversable')),
+						new Identical(
+							$haystack->value,
+							new FuncCall(new Name('array_filter'), [
+								$haystack,
+								new Arg(new Expr\Closure([
+									'static' => true,
+									'params' => [
+										new Param(new Expr\Variable('_')),
+									],
+									'stmts' => [
+										new Stmt\Return_(
+											new FuncCall(new Name('is_a'), [new Arg(new Expr\Variable('_')), $className])
+										),
+									],
+								])),
+							])
+						)
+					);
+				},
 			];
 		}
 
diff --git a/tests/Type/PHPUnit/data/assert-function.php b/tests/Type/PHPUnit/data/assert-function.php
index 117179a..af87a92 100644
--- a/tests/Type/PHPUnit/data/assert-function.php
+++ b/tests/Type/PHPUnit/data/assert-function.php
@@ -4,6 +4,7 @@
 
 use function PHPStan\Testing\assertType;
 use function PHPUnit\Framework\assertArrayHasKey;
+use function PHPUnit\Framework\assertContainsOnlyInstancesOf;
 use function PHPUnit\Framework\assertEmpty;
 use function PHPUnit\Framework\assertInstanceOf;
 use function PHPUnit\Framework\assertObjectHasAttribute;
@@ -43,4 +44,13 @@ public function testEmpty($a): void
 		assertType("0|0.0|''|'0'|array{}|Countable|EmptyIterator|false|null", $a);
 	}
 
+	public function containsOnlyInstancesOf(array $a, \Traversable $b): void
+	{
+		assertContainsOnlyInstancesOf(\stdClass::class, $a);
+		assertType('array<stdClass>', $a);
+
+		assertContainsOnlyInstancesOf(\stdClass::class, $b);
+		assertType('Traversable', $b);
+	}
+
 }