diff --git a/NEWS b/NEWS
index 29bb88d40ec0..287f2b1d0a4a 100644
--- a/NEWS
+++ b/NEWS
@@ -8,6 +8,8 @@ PHP                                                                        NEWS
     of class names in implements, catch, and trait use lists. (Jorg Sowa)
   . Fixed AST export dropping the braces of dynamic class constant fetches.
     (Jorg Sowa)
+  . Fixed bug GH-25238 (ReflectionProperty::isInitialized() skips lazy init once
+    cached). (lazerg)
 
 - Curl:
   . Fixed curl_reset() retaining CURLOPT_PRIVATE after reset. (lacatoire)
diff --git a/ext/reflection/php_reflection.c b/ext/reflection/php_reflection.c
index ea78bf601d48..b799b0347645 100644
--- a/ext/reflection/php_reflection.c
+++ b/ext/reflection/php_reflection.c
@@ -6488,7 +6488,9 @@ ZEND_METHOD(ReflectionProperty, isInitialized)
 
 			if (EXPECTED(IS_VALID_PROPERTY_OFFSET(prop_offset))) {
 				zval *value = OBJ_PROP(Z_OBJ_P(object), prop_offset);
-				RETURN_BOOL(!Z_ISUNDEF_P(value));
+				if (EXPECTED(!Z_ISUNDEF_P(value) || !zend_object_is_lazy(Z_OBJ_P(object)))) {
+					RETURN_BOOL(!Z_ISUNDEF_P(value));
+				}
 			}
 		}
 
diff --git a/ext/reflection/tests/gh25238.phpt b/ext/reflection/tests/gh25238.phpt
new file mode 100644
index 000000000000..24c4b754cabc
--- /dev/null
+++ b/ext/reflection/tests/gh25238.phpt
@@ -0,0 +1,74 @@
+--TEST--
+GH-25238 (ReflectionProperty::isInitialized() does not initialize lazy objects once its cache slot is warm)
+--FILE--
+<?php
+
+class Line {
+    public string $label = '';
+}
+
+class NoDefault {
+    public string $label;
+}
+
+$class = new ReflectionClass(Line::class);
+$property = new ReflectionProperty(Line::class, 'label');
+
+foreach (['first', 'second', 'third'] as $label) {
+    $line = $class->newLazyGhost(function (Line $line) use ($label) {
+        $line->label = $label;
+    });
+    var_dump($property->isInitialized($line));
+    var_dump($class->isUninitializedLazyObject($line));
+    var_dump($line->label);
+}
+
+foreach (['first', 'second'] as $label) {
+    $line = $class->newLazyProxy(function () use ($label) {
+        $line = new Line();
+        $line->label = $label;
+        return $line;
+    });
+    var_dump($property->isInitialized($line));
+    var_dump($class->isUninitializedLazyObject($line));
+    var_dump($property->isInitialized($line));
+    var_dump($line->label);
+}
+
+$line = new Line();
+unset($line->label);
+var_dump($property->isInitialized($line));
+
+$class = new ReflectionClass(NoDefault::class);
+$property = new ReflectionProperty(NoDefault::class, 'label');
+
+for ($i = 0; $i < 2; $i++) {
+    $object = $class->newLazyGhost(function (NoDefault $object) {});
+    var_dump($property->isInitialized($object));
+    var_dump($class->isUninitializedLazyObject($object));
+}
+
+?>
+--EXPECT--
+bool(true)
+bool(false)
+string(5) "first"
+bool(true)
+bool(false)
+string(6) "second"
+bool(true)
+bool(false)
+string(5) "third"
+bool(true)
+bool(false)
+bool(true)
+string(5) "first"
+bool(true)
+bool(false)
+bool(true)
+string(6) "second"
+bool(false)
+bool(false)
+bool(false)
+bool(false)
+bool(false)