diff --git a/Userland/Libraries/LibC/bits/mutex_locker.h b/Userland/Libraries/LibC/bits/mutex_locker.h
new file mode 100644
index 0000000000..e3771c3818
--- /dev/null
+++ b/Userland/Libraries/LibC/bits/mutex_locker.h
@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <pthread.h>
+
+// We don't want to bring LibThreading headers into LibC, so we use plain
+// pthread mutexes and this RAII guard.
+namespace LibC {
+
+class [[nodiscard]] MutexLocker {
+public:
+    explicit MutexLocker(pthread_mutex_t& mutex)
+        : m_mutex(mutex)
+    {
+        lock();
+    }
+
+    ~MutexLocker()
+    {
+        unlock();
+    }
+
+    void lock() { pthread_mutex_lock(&m_mutex); }
+    void unlock() { pthread_mutex_unlock(&m_mutex); }
+
+private:
+    pthread_mutex_t& m_mutex;
+};
+
+}
diff --git a/Userland/Libraries/LibC/bits/stdio_file_implementation.h b/Userland/Libraries/LibC/bits/stdio_file_implementation.h
index 48029992ad..98e4820dcf 100644
--- a/Userland/Libraries/LibC/bits/stdio_file_implementation.h
+++ b/Userland/Libraries/LibC/bits/stdio_file_implementation.h
@@ -5,6 +5,7 @@
  */
 
 #include <AK/Array.h>
+#include <AK/IntrusiveList.h>
 #include <AK/Types.h>
 #include <LibC/bits/FILE.h>
 #include <LibC/bits/pthread_integration.h>
@@ -127,6 +128,10 @@ private:
     pid_t m_popen_child { -1 };
     Buffer m_buffer;
     __pthread_mutex_t m_mutex;
+    IntrusiveListNode<FILE> m_list_node;
+
+public:
+    using List = IntrusiveList<&FILE::m_list_node>;
 };
 
 class ScopedFileLock {
diff --git a/Userland/Libraries/LibC/stdio.cpp b/Userland/Libraries/LibC/stdio.cpp
index 861e20e5ec..4fbd70e8aa 100644
--- a/Userland/Libraries/LibC/stdio.cpp
+++ b/Userland/Libraries/LibC/stdio.cpp
@@ -11,6 +11,7 @@
 #include <AK/ScopedValueRollback.h>
 #include <AK/StdLibExtras.h>
 #include <AK/String.h>
+#include <LibC/bits/mutex_locker.h>
 #include <LibC/bits/stdio_file_implementation.h>
 #include <assert.h>
 #include <errno.h>
@@ -26,6 +27,15 @@
 #include <syscall.h>
 #include <unistd.h>
 
+static constinit pthread_mutex_t s_open_streams_lock = __PTHREAD_MUTEX_INITIALIZER;
+
+// The list of open files is initialized in __stdio_init.
+// We cannot rely on global constructors to initialize it, because it must
+// be initialized before other global constructors run. Similarly, we cannot
+// allow global destructors to destruct it.
+alignas(FILE::List) static u8 s_open_streams_storage[sizeof(FILE::List)];
+static FILE::List* const s_open_streams = reinterpret_cast<FILE::List*>(s_open_streams_storage);
+
 FILE::~FILE()
 {
     bool already_closed = m_fd == -1;
@@ -34,9 +44,13 @@ FILE::~FILE()
 
 FILE* FILE::create(int fd, int mode)
 {
-    void* file = calloc(1, sizeof(FILE));
-    new (file) FILE(fd, mode);
-    return (FILE*)file;
+    void* file_location = calloc(1, sizeof(FILE));
+    if (file_location == nullptr)
+        return nullptr;
+    auto* file = new (file_location) FILE(fd, mode);
+    LibC::MutexLocker locker(s_open_streams_lock);
+    s_open_streams->append(*file);
+    return file;
 }
 
 bool FILE::close()
@@ -517,10 +531,14 @@ FILE* stderr = reinterpret_cast<FILE*>(&default_streams[2]);
 
 void __stdio_init()
 {
+    new (s_open_streams) FILE::List();
     new (stdin) FILE(0, O_RDONLY);
     new (stdout) FILE(1, O_WRONLY);
     new (stderr) FILE(2, O_WRONLY);
     stderr->setbuf(nullptr, _IONBF, 0);
+    s_open_streams->append(*stdin);
+    s_open_streams->append(*stdout);
+    s_open_streams->append(*stderr);
     __stdio_is_initialized = true;
 }
 
@@ -568,8 +586,13 @@ int feof(FILE* stream)
 int fflush(FILE* stream)
 {
     if (!stream) {
-        dbgln("FIXME: fflush(nullptr) should flush all open streams");
-        return 0;
+        int rc = 0;
+        LibC::MutexLocker locker(s_open_streams_lock);
+        for (auto& file : *s_open_streams) {
+            ScopedFileLock lock(&file);
+            rc = file.flush() ? rc : EOF;
+        }
+        return rc;
     }
     ScopedFileLock lock(stream);
     return stream->flush() ? 0 : EOF;
@@ -1070,6 +1093,10 @@ int fclose(FILE* stream)
     }
     ScopedValueRollback errno_restorer(errno);
 
+    {
+        LibC::MutexLocker locker(s_open_streams_lock);
+        s_open_streams->remove(*stream);
+    }
     stream->~FILE();
     if (!is_default_stream(stream))
         free(stream);