diff --git a/Userland/Libraries/LibC/bits/pthread_integration.h b/Userland/Libraries/LibC/bits/pthread_integration.h
index b74462afd2..1d37ee2d44 100644
--- a/Userland/Libraries/LibC/bits/pthread_integration.h
+++ b/Userland/Libraries/LibC/bits/pthread_integration.h
@@ -21,6 +21,7 @@ void __pthread_fork_atfork_register_child(void (*)(void));
 int __pthread_mutex_init(pthread_mutex_t*, const pthread_mutexattr_t*);
 int __pthread_mutex_lock(pthread_mutex_t*);
 int __pthread_mutex_trylock(pthread_mutex_t*);
+int __pthread_mutex_lock_pessimistic_np(pthread_mutex_t*);
 int __pthread_mutex_unlock(pthread_mutex_t*);
 
 typedef void (*KeyDestructor)(void*);
diff --git a/Userland/Libraries/LibC/pthread_integration.cpp b/Userland/Libraries/LibC/pthread_integration.cpp
index ebe7b232ca..e2f10cdda0 100644
--- a/Userland/Libraries/LibC/pthread_integration.cpp
+++ b/Userland/Libraries/LibC/pthread_integration.cpp
@@ -167,6 +167,22 @@ int __pthread_mutex_lock(pthread_mutex_t* mutex)
 
 int pthread_mutex_lock(pthread_mutex_t*) __attribute__((weak, alias("__pthread_mutex_lock")));
 
+int __pthread_mutex_lock_pessimistic_np(pthread_mutex_t* mutex)
+{
+    // Same as pthread_mutex_lock(), but always set MUTEX_LOCKED_NEED_TO_WAKE,
+    // and also don't bother checking for already owning the mutex recursively,
+    // because we know we don't. Used in the condition variable implementation.
+    u32 value = AK::atomic_exchange(&mutex->lock, MUTEX_LOCKED_NEED_TO_WAKE, AK::memory_order_acquire);
+    while (value != MUTEX_UNLOCKED) {
+        futex_wait(&mutex->lock, value, nullptr, 0);
+        value = AK::atomic_exchange(&mutex->lock, MUTEX_LOCKED_NEED_TO_WAKE, AK::memory_order_acquire);
+    }
+
+    AK::atomic_store(&mutex->owner, __pthread_self(), AK::memory_order_relaxed);
+    mutex->level = 0;
+    return 0;
+}
+
 int __pthread_mutex_unlock(pthread_mutex_t* mutex)
 {
     if (mutex->type == __PTHREAD_MUTEX_RECURSIVE && mutex->level > 0) {