diff --git a/BUILD.gn b/BUILD.gn
index c0b47d5d11..60e8fa457d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -555,6 +555,7 @@ if (rtc_include_tests && !build_with_chromium) {
       "rtc_base:untyped_function_unittest",
       "rtc_base:weak_ptr_unittests",
       "rtc_base/experiments:experiments_unittests",
+      "rtc_base/system:file_wrapper_unittests",
       "rtc_base/task_utils:pending_task_safety_flag_unittests",
       "rtc_base/task_utils:to_queued_task_unittests",
       "sdk:sdk_tests",
diff --git a/rtc_base/system/BUILD.gn b/rtc_base/system/BUILD.gn
index 9f83c629e2..c604796e60 100644
--- a/rtc_base/system/BUILD.gn
+++ b/rtc_base/system/BUILD.gn
@@ -32,6 +32,19 @@ rtc_library("file_wrapper") {
   ]
 }
 
+if (rtc_include_tests) {
+  rtc_library("file_wrapper_unittests") {
+    testonly = true
+    sources = [ "file_wrapper_unittest.cc" ]
+    deps = [
+      ":file_wrapper",
+      "//rtc_base:checks",
+      "//test:fileutils",
+      "//test:test_support",
+    ]
+  }
+}
+
 rtc_source_set("ignore_warnings") {
   sources = [ "ignore_warnings.h" ]
 }
diff --git a/rtc_base/system/file_wrapper.cc b/rtc_base/system/file_wrapper.cc
index 2828790e09..3e49315793 100644
--- a/rtc_base/system/file_wrapper.cc
+++ b/rtc_base/system/file_wrapper.cc
@@ -89,6 +89,22 @@ bool FileWrapper::SeekTo(int64_t position) {
   return fseek(file_, rtc::checked_cast<long>(position), SEEK_SET) == 0;
 }
 
+long FileWrapper::FileSize() {
+  if (file_ == nullptr)
+    return -1;
+  long original_position = ftell(file_);
+  if (original_position < 0)
+    return -1;
+  int seek_error = fseek(file_, 0, SEEK_END);
+  if (seek_error)
+    return -1;
+  long file_size = ftell(file_);
+  seek_error = fseek(file_, original_position, SEEK_SET);
+  if (seek_error)
+    return -1;
+  return file_size;
+}
+
 bool FileWrapper::Flush() {
   RTC_DCHECK(file_);
   return fflush(file_) == 0;
diff --git a/rtc_base/system/file_wrapper.h b/rtc_base/system/file_wrapper.h
index 42c463cb15..0b293d9a80 100644
--- a/rtc_base/system/file_wrapper.h
+++ b/rtc_base/system/file_wrapper.h
@@ -38,7 +38,6 @@ class FileWrapper final {
   static FileWrapper OpenReadOnly(const std::string& file_name_utf8);
   static FileWrapper OpenWriteOnly(const char* file_name_utf8,
                                    int* error = nullptr);
-
   static FileWrapper OpenWriteOnly(const std::string& file_name_utf8,
                                    int* error = nullptr);
 
@@ -87,6 +86,11 @@ class FileWrapper final {
   // Seek to given position.
   bool SeekTo(int64_t position);
 
+  // Returns the file size or -1 if a size could not be determined.
+  // (A file size might not exists for non-seekable files or file-like
+  // objects, for example /dev/tty on unix.)
+  long FileSize();
+
   // Returns number of bytes read. Short count indicates EOF or error.
   size_t Read(void* buf, size_t length);
 
diff --git a/rtc_base/system/file_wrapper_unittest.cc b/rtc_base/system/file_wrapper_unittest.cc
new file mode 100644
index 0000000000..980b565c73
--- /dev/null
+++ b/rtc_base/system/file_wrapper_unittest.cc
@@ -0,0 +1,69 @@
+/*
+ *  Copyright 2021 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "rtc_base/system/file_wrapper.h"
+
+#include "rtc_base/checks.h"
+#include "test/gtest.h"
+#include "test/testsupport/file_utils.h"
+
+namespace webrtc {
+
+TEST(FileWrapper, FileSize) {
+  auto test_info = ::testing::UnitTest::GetInstance()->current_test_info();
+  std::string test_name =
+      std::string(test_info->test_case_name()) + "_" + test_info->name();
+  std::replace(test_name.begin(), test_name.end(), '/', '_');
+  const std::string temp_filename = test::OutputPath() + test_name;
+
+  // Write
+  {
+    FileWrapper file = FileWrapper::OpenWriteOnly(temp_filename);
+    ASSERT_TRUE(file.is_open());
+    EXPECT_EQ(file.FileSize(), 0);
+
+    EXPECT_TRUE(file.Write("foo", 3));
+    EXPECT_EQ(file.FileSize(), 3);
+
+    // FileSize() doesn't change the file size.
+    EXPECT_EQ(file.FileSize(), 3);
+
+    // FileSize() doesn't move the write position.
+    EXPECT_TRUE(file.Write("bar", 3));
+    EXPECT_EQ(file.FileSize(), 6);
+  }
+
+  // Read
+  {
+    FileWrapper file = FileWrapper::OpenReadOnly(temp_filename);
+    ASSERT_TRUE(file.is_open());
+    EXPECT_EQ(file.FileSize(), 6);
+
+    char buf[10];
+    size_t bytes_read = file.Read(buf, 3);
+    EXPECT_EQ(bytes_read, 3u);
+    EXPECT_EQ(memcmp(buf, "foo", 3), 0);
+
+    // FileSize() doesn't move the read position.
+    EXPECT_EQ(file.FileSize(), 6);
+
+    // Attempting to read past the end reads what is available
+    // and sets the EOF flag.
+    bytes_read = file.Read(buf, 5);
+    EXPECT_EQ(bytes_read, 3u);
+    EXPECT_EQ(memcmp(buf, "bar", 3), 0);
+    EXPECT_TRUE(file.ReadEof());
+  }
+
+  // Clean up temporary file.
+  remove(temp_filename.c_str());
+}
+
+}  // namespace webrtc