diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index 776641b347e6c..7a084d114b185 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -308,6 +308,43 @@ private:
   state_type __st_;
   state_type __st_last_;
   ios_base::openmode __om_;
+  // There have been no file operations yet, which allows setting unbuffered
+  // I/O mode.
+  static const ios_base::openmode __no_io_operations = ios_base::trunc;
+  // Unbuffered I/O mode has been requested.
+  static const ios_base::openmode __use_unbuffered_io = ios_base::ate;
+  // Used to track the currently used mode and track whether the output should
+  // be unbuffered.
+  // [filebuf.virtuals]/12
+  //   If setbuf(0, 0) is called on a stream before any I/O has occurred on
+  //   that stream, the stream becomes unbuffered. Otherwise the results are
+  //   implementation-defined.
+  // This allows calling setbuf(0, 0)
+  // - before opening a file,
+  // - after opening a file, before
+  //   - a read
+  //   - a write
+  //   - a seek.
+  // Note that opening a file with ios_base::ate does a seek operation.
+  // Normally underflow, overflow, and sync change this flag to ios_base::in,
+  // ios_base_out, or 0.
+  //
+  // The ios_base::trunc and ios_base::ate flags are not used in __cm_. They
+  // are used to track the state of the unbuffered request. For readability
+  // they have the aliases __no_io_operations and __use_unbuffered_io
+  // respectively.
+  //
+  // The __no_io_operations and __use_unbuffered_io flags are used in the
+  // following way:
+  // - __no_io_operations is set upon construction to indicate the unbuffered
+  //   state can be set.
+  // - When requesting unbuffered output:
+  //   - If the file is open it sets the mode.
+  //   - Else places a request by adding the __use_unbuffered_io flag.
+  // - When a file is opened it checks whether both __no_io_operations and
+  //   __use_unbuffered_io are set. If so switches to unbuffered mode.
+  // - All file I/O operations change the mode effectively clearing the
+  //   __no_io_operations and __use_unbuffered_io flags.
   ios_base::openmode __cm_;
   bool __owns_eb_;
   bool __owns_ib_;
@@ -327,7 +364,13 @@ private:
       return nullptr;
 
     __om_ = __mode;
+    if (__cm_ == (__no_io_operations | __use_unbuffered_io)) {
+      std::setbuf(__file_, nullptr);
+      __cm_ = 0;
+    }
+
     if (__mode & ios_base::ate) {
+      __cm_ = 0;
       if (fseek(__file_, 0, SEEK_END)) {
         fclose(__file_);
         __file_ = nullptr;
@@ -337,6 +380,20 @@ private:
 
     return this;
   }
+
+  // If the file is already open, switch to unbuffered mode. Otherwise, record
+  // the request to use unbuffered mode so that we use that mode when we
+  // eventually open the file.
+  _LIBCPP_HIDE_FROM_ABI void __request_unbuffered_mode(char_type* __s, streamsize __n) {
+    if (__cm_ == __no_io_operations && __s == nullptr && __n == 0) {
+      if (__file_) {
+        std::setbuf(__file_, nullptr);
+        __cm_ = 0;
+      } else {
+        __cm_ = __no_io_operations | __use_unbuffered_io;
+      }
+    }
+  }
 };
 
 template <class _CharT, class _Traits>
@@ -352,7 +409,7 @@ basic_filebuf<_CharT, _Traits>::basic_filebuf()
       __st_(),
       __st_last_(),
       __om_(0),
-      __cm_(0),
+      __cm_(__no_io_operations),
       __owns_eb_(false),
       __owns_ib_(false),
       __always_noconv_(false) {
@@ -810,6 +867,7 @@ template <class _CharT, class _Traits>
 basic_streambuf<_CharT, _Traits>* basic_filebuf<_CharT, _Traits>::setbuf(char_type* __s, streamsize __n) {
   this->setg(nullptr, nullptr, nullptr);
   this->setp(nullptr, nullptr);
+  __request_unbuffered_mode(__s, __n);
   if (__owns_eb_)
     delete[] __extbuf_;
   if (__owns_ib_)
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/setbuf.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/setbuf.pass.cpp
new file mode 100644
index 0000000000000..8bcce28162033
--- /dev/null
+++ b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/setbuf.pass.cpp
@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <fstream>
+
+// basic_streambuf<charT, traits>* setbuf(char_type* s, streamsize n) override;
+
+#include <fstream>
+#include <cstddef>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class CharT>
+static std::size_t file_size(const char* filename) {
+  FILE* f = std::fopen(filename, "rb");
+  std::fseek(f, 0, SEEK_END);
+  long result = std::ftell(f);
+  std::fclose(f);
+  return result;
+}
+
+// Helper class to expose some protected std::basic_filebuf<CharT> members.
+template <class CharT>
+struct filebuf : public std::basic_filebuf<CharT> {
+  CharT* base() { return this->pbase(); }
+  CharT* ptr() { return this->pptr(); }
+};
+
+template <class CharT>
+static void buffered_request() {
+  filebuf<CharT> buffer;
+
+  CharT b[10] = {0};
+  assert(buffer.pubsetbuf(b, 10) == &buffer);
+
+  buffer.open("test.dat", std::ios_base::out);
+  buffer.sputc(CharT('a'));
+  assert(b[0] == 'a');
+
+  buffer.close();
+  assert(file_size<CharT>("test.dat") == 1);
+}
+
+template <class CharT>
+static void unbuffered_request_before_open() {
+  filebuf<CharT> buffer;
+
+  assert(buffer.pubsetbuf(nullptr, 0) == &buffer);
+  assert(buffer.base() == nullptr);
+  assert(buffer.ptr() == nullptr);
+
+  buffer.open("test.dat", std::ios_base::out);
+  assert(buffer.base() == nullptr);
+  assert(buffer.ptr() == nullptr);
+
+  buffer.sputc(CharT('a'));
+  assert(buffer.base() == nullptr);
+  assert(buffer.ptr() == nullptr);
+
+  assert(file_size<CharT>("test.dat") == 1);
+}
+
+template <class CharT>
+static void unbuffered_request_after_open() {
+  filebuf<CharT> buffer;
+
+  buffer.open("test.dat", std::ios_base::out);
+
+  assert(buffer.pubsetbuf(nullptr, 0) == &buffer);
+  assert(buffer.base() == nullptr);
+  assert(buffer.ptr() == nullptr);
+
+  buffer.sputc(CharT('a'));
+  assert(buffer.base() == nullptr);
+  assert(buffer.ptr() == nullptr);
+
+  assert(file_size<CharT>("test.dat") == 1);
+}
+
+template <class CharT>
+static void unbuffered_request_after_open_ate() {
+  filebuf<CharT> buffer;
+
+  buffer.open("test.dat", std::ios_base::out | std::ios_base::ate);
+
+  assert(buffer.pubsetbuf(nullptr, 0) == &buffer);
+
+  buffer.sputc(CharT('a'));
+  assert(file_size<CharT>("test.dat") <= 1);
+  // on libc++ buffering is used by default.
+  LIBCPP_ASSERT(file_size<CharT>("test.dat") == 0);
+
+  buffer.close();
+  assert(file_size<CharT>("test.dat") == 1);
+}
+
+template <class CharT>
+static void test() {
+  buffered_request<CharT>();
+
+  unbuffered_request_before_open<CharT>();
+  unbuffered_request_after_open<CharT>();
+  unbuffered_request_after_open_ate<CharT>();
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}