diff --git a/src/Rules/Properties/ReadOnlyPropertyAssignRule.php b/src/Rules/Properties/ReadOnlyPropertyAssignRule.php
index 4e9673070f..a623d48536 100644
--- a/src/Rules/Properties/ReadOnlyPropertyAssignRule.php
+++ b/src/Rules/Properties/ReadOnlyPropertyAssignRule.php
@@ -2,14 +2,18 @@
 
 namespace PHPStan\Rules\Properties;
 
+use ArrayAccess;
 use PhpParser\Node;
 use PHPStan\Analyser\Scope;
+use PHPStan\Node\Expr\SetOffsetValueTypeExpr;
+use PHPStan\Node\Expr\UnsetOffsetExpr;
 use PHPStan\Node\PropertyAssignNode;
 use PHPStan\Reflection\ConstructorsHelper;
 use PHPStan\Reflection\MethodReflection;
 use PHPStan\Rules\Rule;
 use PHPStan\Rules\RuleErrorBuilder;
 use PHPStan\ShouldNotHappenException;
+use PHPStan\Type\ObjectType;
 use PHPStan\Type\TypeUtils;
 use function in_array;
 use function sprintf;
@@ -89,6 +93,14 @@ public function processNode(Node $node, Scope $scope): array
 				continue;
 			}
 
+			$expr = $node->getAssignedExpr();
+			if (
+				(new ObjectType(ArrayAccess::class))->isSuperTypeOf($propertyReflection->getNativeType())->yes()
+				 && (($expr instanceof SetOffsetValueTypeExpr) || ($expr instanceof UnsetOffsetExpr))
+			) {
+				continue;
+			}
+
 			$errors[] = RuleErrorBuilder::message(sprintf('Readonly property %s::$%s is assigned outside of the constructor.', $declaringClass->getDisplayName(), $propertyReflection->getName()))
 				->identifier('property.readOnlyAssignNotInConstructor')
 				->build();
diff --git a/tests/PHPStan/Rules/Properties/ReadOnlyPropertyAssignRuleTest.php b/tests/PHPStan/Rules/Properties/ReadOnlyPropertyAssignRuleTest.php
index 966f8e9e41..a37df3bb13 100644
--- a/tests/PHPStan/Rules/Properties/ReadOnlyPropertyAssignRuleTest.php
+++ b/tests/PHPStan/Rules/Properties/ReadOnlyPropertyAssignRuleTest.php
@@ -168,4 +168,18 @@ public function testBug6773(): void
 		]);
 	}
 
+	public function testBug8929(): void
+	{
+		if (PHP_VERSION_ID < 80100) {
+			self::markTestSkipped('Test requires PHP 8.1');
+		}
+
+		$this->analyse([__DIR__ . '/data/bug-8929.php'], [
+			[
+				'Readonly property Bug8929\Test::$cache is assigned outside of the constructor.',
+				19,
+			],
+		]);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Properties/data/bug-8929.php b/tests/PHPStan/Rules/Properties/data/bug-8929.php
new file mode 100644
index 0000000000..006f3aecf5
--- /dev/null
+++ b/tests/PHPStan/Rules/Properties/data/bug-8929.php
@@ -0,0 +1,21 @@
+<?php declare(strict_types = 1); // lint >= 8.1
+
+namespace Bug8929;
+
+class Test
+{
+    /** @var \WeakMap<object, mixed> */
+    protected readonly \WeakMap $cache;
+
+    public function __construct()
+    {
+        $this->cache = new \WeakMap();
+    }
+
+    public function add(object $key, mixed $value): void
+    {
+        $this->cache[$key] = $value; // valid offset access
+        unset($this->cache[$key]);   // valid offset access
+        $this->cache = new \WeakMap(); // reassigning is invalid however
+    }
+}