diff --git a/src/Rules/Variables/UnsetRule.php b/src/Rules/Variables/UnsetRule.php
index dc75eb024e..8240e35e95 100644
--- a/src/Rules/Variables/UnsetRule.php
+++ b/src/Rules/Variables/UnsetRule.php
@@ -5,6 +5,7 @@
 use PhpParser\Node;
 use PHPStan\Analyser\Scope;
 use PHPStan\Rules\IdentifierRuleError;
+use PHPStan\Rules\Properties\PropertyReflectionFinder;
 use PHPStan\Rules\Rule;
 use PHPStan\Rules\RuleErrorBuilder;
 use PHPStan\Type\VerbosityLevel;
@@ -17,6 +18,12 @@
 final class UnsetRule implements Rule
 {
 
+	public function __construct(
+		private PropertyReflectionFinder $propertyReflectionFinder,
+	)
+	{
+	}
+
 	public function getNodeType(): string
 	{
 		return Node\Stmt\Unset_::class;
@@ -69,6 +76,33 @@ private function canBeUnset(Node $node, Scope $scope): ?IdentifierRuleError
 			}
 
 			return $this->canBeUnset($node->var, $scope);
+		} elseif (
+			$node instanceof Node\Expr\PropertyFetch
+			&& $node->name instanceof Node\Identifier
+		) {
+			$foundPropertyReflection = $this->propertyReflectionFinder->findPropertyReflectionFromNode($node, $scope);
+			if ($foundPropertyReflection === null) {
+				return null;
+			}
+
+			$propertyReflection = $foundPropertyReflection->getNativeReflection();
+			if ($propertyReflection === null) {
+				return null;
+			}
+
+			if ($propertyReflection->isReadOnly() || $propertyReflection->isReadOnlyByPhpDoc()) {
+				return RuleErrorBuilder::message(
+					sprintf(
+						'Cannot unset %s %s::$%s property.',
+						$propertyReflection->isReadOnly() ? 'readonly' : '@readonly',
+						$propertyReflection->getDeclaringClass()->getDisplayName(),
+						$foundPropertyReflection->getName(),
+					),
+				)
+					->line($node->getStartLine())
+					->identifier($propertyReflection->isReadOnly() ? 'unset.readOnlyProperty' : 'unset.readOnlyPropertyByPhpDoc')
+					->build();
+			}
 		}
 
 		return null;
diff --git a/tests/PHPStan/Rules/Variables/UnsetRuleTest.php b/tests/PHPStan/Rules/Variables/UnsetRuleTest.php
index 036d09c974..0b5861c1c0 100644
--- a/tests/PHPStan/Rules/Variables/UnsetRuleTest.php
+++ b/tests/PHPStan/Rules/Variables/UnsetRuleTest.php
@@ -2,6 +2,7 @@
 
 namespace PHPStan\Rules\Variables;
 
+use PHPStan\Rules\Properties\PropertyReflectionFinder;
 use PHPStan\Rules\Rule;
 use PHPStan\Testing\RuleTestCase;
 
@@ -13,7 +14,7 @@ class UnsetRuleTest extends RuleTestCase
 
 	protected function getRule(): Rule
 	{
-		return new UnsetRule();
+		return new UnsetRule(self::getContainer()->getByType(PropertyReflectionFinder::class));
 	}
 
 	public function testUnsetRule(): void
@@ -91,4 +92,34 @@ public function testBug4565(): void
 		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-4565.php'], []);
 	}
 
+	public function testBug12421(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-12421.php'], [
+			[
+				'Cannot unset readonly Bug12421\NativeReadonlyClass::$y property.',
+				11,
+			],
+			[
+				'Cannot unset readonly Bug12421\NativeReadonlyProperty::$y property.',
+				15,
+			],
+			[
+				'Cannot unset @readonly Bug12421\PhpdocReadonlyClass::$y property.',
+				19,
+			],
+			[
+				'Cannot unset @readonly Bug12421\PhpdocReadonlyProperty::$y property.',
+				23,
+			],
+			[
+				'Cannot unset @readonly Bug12421\PhpdocImmutableClass::$y property.',
+				27,
+			],
+			[
+				'Cannot unset readonly Bug12421\NativeReadonlyProperty::$y property.',
+				34,
+			],
+		]);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Variables/data/bug-12421.php b/tests/PHPStan/Rules/Variables/data/bug-12421.php
new file mode 100644
index 0000000000..eb0c53f755
--- /dev/null
+++ b/tests/PHPStan/Rules/Variables/data/bug-12421.php
@@ -0,0 +1,108 @@
+<?php // lint >= 8.2
+
+namespace Bug12421;
+
+function doFoo() {
+	$x = new RegularProperty();
+	unset($x->y);
+	var_dump($x->y);
+
+	$x = new NativeReadonlyClass();
+	unset($x->y);
+	var_dump($x->y);
+
+	$x = new NativeReadonlyProperty();
+	unset($x->y);
+	var_dump($x->y);
+
+	$x = new PhpdocReadonlyClass();
+	unset($x->y);
+	var_dump($x->y);
+
+	$x = new PhpdocReadonlyProperty();
+	unset($x->y);
+	var_dump($x->y);
+
+	$x = new PhpdocImmutableClass();
+	unset($x->y);
+	var_dump($x->y);
+
+	$x = new \stdClass();
+	unset($x->y);
+
+	$x = new NativeReadonlyPropertySubClass();
+	unset($x->y);
+	var_dump($x->y);
+}
+
+readonly class NativeReadonlyClass
+{
+	public Y $y;
+
+	public function __construct()
+	{
+		$this->y = new Y();
+	}
+}
+
+class NativeReadonlyProperty
+{
+	public readonly Y $y;
+
+	public function __construct()
+	{
+		$this->y = new Y();
+	}
+}
+
+/** @readonly */
+class PhpdocReadonlyClass
+{
+	public Y $y;
+
+	public function __construct()
+	{
+		$this->y = new Y();
+	}
+}
+
+class PhpdocReadonlyProperty
+{
+	/** @readonly */
+	public Y $y;
+
+	public function __construct()
+	{
+		$this->y = new Y();
+	}
+}
+
+/** @immutable  */
+class PhpdocImmutableClass
+{
+	public Y $y;
+
+	public function __construct()
+	{
+		$this->y = new Y();
+	}
+}
+
+class RegularProperty
+{
+	public Y $y;
+
+	public function __construct()
+	{
+		$this->y = new Y();
+	}
+}
+
+class NativeReadonlyPropertySubClass extends NativeReadonlyProperty
+{
+}
+
+class Y
+{
+}
+