diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -19,6 +19,7 @@
     
     # fcntl.h entrypoints
     libc.src.fcntl.creat
+    libc.src.fcntl.fchmodat
     libc.src.fcntl.open
     libc.src.fcntl.openat
 
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -19,6 +19,7 @@
 
     # fcntl.h entrypoints
     libc.src.fcntl.creat
+    libc.src.fcntl.fchmodat
     libc.src.fcntl.open
     libc.src.fcntl.openat
 
diff --git a/libc/spec/posix.td b/libc/spec/posix.td
--- a/libc/spec/posix.td
+++ b/libc/spec/posix.td
@@ -154,6 +154,11 @@
           RetValSpec<IntType>,
           [ArgSpec<ConstCharPtr>, ArgSpec<ModeTType>]
       >,
+      FunctionSpec<
+          "fchmodat",
+          RetValSpec<IntType>,
+          [ArgSpec<IntType>, ArgSpec<ConstCharPtr>, ArgSpec<ModeTType>, ArgSpec<IntType>]
+      >,
       FunctionSpec<
           "open",
           RetValSpec<IntType>,
@@ -406,12 +411,12 @@
         FunctionSpec<
           "chmod",
           RetValSpec<IntType>,
-          [ArgSpec<ConstCharPtr>]
+          [ArgSpec<ConstCharPtr>, ArgSpec<ModeTType>]
         >,
         FunctionSpec<
           "fchmod",
           RetValSpec<IntType>,
-          [ArgSpec<IntType>]
+          [ArgSpec<IntType>, ArgSpec<ModeTType>]
         >,
         FunctionSpec<
             "mkdir",
diff --git a/libc/src/fcntl/CMakeLists.txt b/libc/src/fcntl/CMakeLists.txt
--- a/libc/src/fcntl/CMakeLists.txt
+++ b/libc/src/fcntl/CMakeLists.txt
@@ -9,6 +9,13 @@
     .${LIBC_TARGET_OS}.creat
 )
 
+add_entrypoint_object(
+  fchmodat
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.fchmodat
+)
+
 add_entrypoint_object(
   open
   ALIAS
diff --git a/libc/src/fcntl/fchmodat.h b/libc/src/fcntl/fchmodat.h
new file mode 100644
--- /dev/null
+++ b/libc/src/fcntl/fchmodat.h
@@ -0,0 +1,20 @@
+//===-- Implementation header for fchmodat ----------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SYS_STAT_FCHMODAT_H
+#define LLVM_LIBC_SRC_SYS_STAT_FCHMODAT_H
+
+#include <sys/stat.h>
+
+namespace __llvm_libc {
+
+int fchmodat(int dirfd, const char *path, mode_t mode, int flags);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SYS_STAT_FCHMODAT_H
diff --git a/libc/src/fcntl/linux/CMakeLists.txt b/libc/src/fcntl/linux/CMakeLists.txt
--- a/libc/src/fcntl/linux/CMakeLists.txt
+++ b/libc/src/fcntl/linux/CMakeLists.txt
@@ -11,6 +11,19 @@
     libc.src.errno.errno
 )
 
+add_entrypoint_object(
+  fchmodat
+  SRCS
+    fchmodat.cpp
+  HDRS
+    ../fchmodat.h
+  DEPENDS
+    libc.include.fcntl
+    libc.include.sys_syscall
+    libc.src.__support.OSUtil.osutil
+    libc.src.errno.errno
+)
+
 add_entrypoint_object(
   open
   SRCS
diff --git a/libc/src/fcntl/linux/fchmodat.cpp b/libc/src/fcntl/linux/fchmodat.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/fcntl/linux/fchmodat.cpp
@@ -0,0 +1,30 @@
+//===-- Linux implementation of fchmodat ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/fcntl/fchmodat.h"
+
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+
+#include <errno.h>
+#include <fcntl.h>       // For mode_t
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(int, fchmodat,
+                   (int dirfd, const char *path, mode_t mode, int flags)) {
+  long ret = __llvm_libc::syscall(SYS_fchmodat, dirfd, path, mode, flags);
+  if (ret < 0) {
+    errno = -ret;
+    return -1;
+  }
+  return 0;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/fcntl/CMakeLists.txt b/libc/test/src/fcntl/CMakeLists.txt
--- a/libc/test/src/fcntl/CMakeLists.txt
+++ b/libc/test/src/fcntl/CMakeLists.txt
@@ -17,6 +17,21 @@
     libc.test.errno_setter_matcher
 )
 
+add_libc_unittest(
+  fchmodat_test
+  SUITE
+    libc_fcntl_unittests
+  SRCS
+    fchmodat_test.cpp
+  DEPENDS
+    libc.include.errno
+    libc.include.fcntl
+    libc.src.fcntl.fchmodat
+    libc.src.fcntl.open
+    libc.src.unistd.close
+    libc.src.unistd.write
+)
+
 add_libc_unittest(
   openat_test
   SUITE
diff --git a/libc/test/src/fcntl/fchmodat_test.cpp b/libc/test/src/fcntl/fchmodat_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/fcntl/fchmodat_test.cpp
@@ -0,0 +1,71 @@
+//===-- Unittests for fchmodat --------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/fcntl/fchmodat.h"
+#include "src/fcntl/open.h"
+#include "src/unistd/close.h"
+#include "src/unistd/write.h"
+#include "test/ErrnoSetterMatcher.h"
+#include "utils/UnitTest/Test.h"
+#include "utils/testutils/FDReader.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+
+TEST(LlvmLibcFchmodatTest, ChangeAndOpen) {
+  using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
+  using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
+
+  // The test file is initially writable. We open it for writing and ensure
+  // that it indeed can be opened for writing. Next, we close the file and
+  // make it readonly using chmod. We test that chmod actually succeeded by
+  // trying to open the file for writing and failing.
+  constexpr const char *TEST_FILE = "testdata/fchmodat.test";
+  constexpr const char *TEST_DIR = "testdata";
+  constexpr const char *TEST_FILE_BASENAME = "fchmodat.test";
+  const char WRITE_DATA[] = "fchmodat test";
+  constexpr ssize_t WRITE_SIZE = ssize_t(sizeof(WRITE_DATA));
+  errno = 0;
+
+  int fd = __llvm_libc::open(TEST_FILE, O_CREAT | O_WRONLY, S_IRWXU);
+  ASSERT_GT(fd, 0);
+  ASSERT_EQ(errno, 0);
+  ASSERT_EQ(__llvm_libc::write(fd, WRITE_DATA, sizeof(WRITE_DATA)), WRITE_SIZE);
+  ASSERT_THAT(__llvm_libc::close(fd), Succeeds(0));
+
+  int dirfd = __llvm_libc::open(TEST_DIR, O_DIRECTORY);
+  ASSERT_GT(dirfd, 0);
+  ASSERT_EQ(errno, 0);
+
+  EXPECT_THAT(__llvm_libc::fchmodat(dirfd, TEST_FILE_BASENAME, S_IRUSR, 0),
+              Succeeds(0));
+
+  // Opening for writing should fail.
+  EXPECT_EQ(__llvm_libc::open(TEST_FILE, O_APPEND | O_WRONLY), -1);
+  EXPECT_NE(errno, 0);
+  errno = 0;
+  // But opening for reading should succeed.
+  fd = __llvm_libc::open(TEST_FILE, O_APPEND | O_RDONLY);
+  EXPECT_GT(fd, 0);
+  EXPECT_EQ(errno, 0);
+
+  EXPECT_THAT(__llvm_libc::close(fd), Succeeds(0));
+  EXPECT_THAT(__llvm_libc::fchmodat(dirfd, TEST_FILE_BASENAME, S_IRWXU, 0),
+              Succeeds(0));
+
+  EXPECT_THAT(__llvm_libc::close(dirfd), Succeeds(0));
+}
+
+TEST(LlvmLibcFchmodatTest, NonExistentFile) {
+  errno = 0;
+  using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
+  ASSERT_THAT(__llvm_libc::fchmodat(AT_FDCWD, "non-existent-file", S_IRUSR, 0),
+              Fails(ENOENT));
+  errno = 0;
+}
diff --git a/libc/test/src/sys/stat/testdata/CMakeLists.txt b/libc/test/src/sys/stat/testdata/CMakeLists.txt
--- a/libc/test/src/sys/stat/testdata/CMakeLists.txt
+++ b/libc/test/src/sys/stat/testdata/CMakeLists.txt
@@ -1,4 +1,5 @@
 # This directory will be used to create test files.
 
 file(GENERATE OUTPUT chmod.test CONTENT "chmod test")
+file(GENERATE OUTPUT fchmodat.test CONTENT "fchmodat test")
 file(GENERATE OUTPUT fchmod.test CONTENT "fchmod test")