diff --git a/libc/config/linux/api.td b/libc/config/linux/api.td
--- a/libc/config/linux/api.td
+++ b/libc/config/linux/api.td
@@ -28,6 +28,13 @@
   }];
 }
 
+def ErrnoMacro : MacroDef<"errno"> {
+  let Defn = [{
+    extern int *__errno_location();
+    #define errno *__errno_location()
+  }];
+}
+
 def MathAPI : PublicAPI<"math.h"> {
   let Functions = [
    "acos",
@@ -83,3 +90,9 @@
     "snprintf",
   ];
 }
+
+def ErrnoAPI : PublicAPI<"errno.h"> {
+  let Macros = [
+    ErrnoMacro,
+  ];
+}
diff --git a/libc/include/CMakeLists.txt b/libc/include/CMakeLists.txt
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@@ -28,3 +28,9 @@
   DEPENDS
     llvm_libc_common_h
 )
+
+add_gen_header(
+  errno_h
+  DEF_FILE errno.h.def
+  GEN_HDR errno.h
+)
diff --git a/libc/include/errno.h.def b/libc/include/errno.h.def
new file mode 100644
--- /dev/null
+++ b/libc/include/errno.h.def
@@ -0,0 +1,14 @@
+//===---------------- C standard library header errno.h ------------------===//
+//
+// 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_ERRNO_H
+#define LLVM_LIBC_ERRNO_H
+
+%%public_api()
+
+#endif // LLVM_LIBC_ERRNO_H
diff --git a/libc/lib/CMakeLists.txt b/libc/lib/CMakeLists.txt
--- a/libc/lib/CMakeLists.txt
+++ b/libc/lib/CMakeLists.txt
@@ -2,8 +2,10 @@
 add_entrypoint_library(
   llvmlibc
   DEPENDS
+    # errno.h entrypoints
+    __errno_location
+
     # string.h entrypoints
-    ## C standard library entrypoints
     strcpy
     strcat
 )
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -170,7 +170,17 @@
       ]
   >;
 
+  HeaderSpec Errno = HeaderSpec<
+      "errno.h",
+      [
+        Macro<"errno">,
+      ],
+      [], // Types
+      []  // Functions
+  >;
+
   let Headers = [
+    Errno,
     Math,
     String,
     StdIO,
diff --git a/libc/src/CMakeLists.txt b/libc/src/CMakeLists.txt
--- a/libc/src/CMakeLists.txt
+++ b/libc/src/CMakeLists.txt
@@ -1,4 +1,5 @@
-add_subdirectory(string)
+add_subdirectory(errno)
 add_subdirectory(math)
+add_subdirectory(string)
 
 add_subdirectory(__support)
diff --git a/libc/src/errno/CMakeLists.txt b/libc/src/errno/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/libc/src/errno/CMakeLists.txt
@@ -0,0 +1,19 @@
+add_entrypoint_object(
+  __errno_location
+  SRCS
+    errno_location.cpp
+  HDRS
+    llvmlibc_errno.h
+)
+
+add_custom_target(libc_errno_unittests)
+
+add_libc_unittest(
+  errno_test
+  SUITE
+    libc_errno_unittests
+  SRCS
+    errno_test.cpp
+  DEPENDS
+    __errno_location
+)
diff --git a/libc/src/errno/errno_location.cpp b/libc/src/errno/errno_location.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/errno/errno_location.cpp
@@ -0,0 +1,22 @@
+//===----------------- Implementation of __errno_location -----------------===//
+//
+// 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/errno/llvmlibc_errno.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+static thread_local int errno = 0;
+
+// __errno_location is not really an entry point but we still want it to behave
+// like an entry point because the errno macro resolves to the C symbol
+// "__errno_location".
+int *LLVM_LIBC_ENTRYPOINT(__errno_location)() { return &errno; }
+
+} // namespace __llvm_libc
diff --git a/libc/src/errno/errno_test.cpp b/libc/src/errno/errno_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/errno/errno_test.cpp
@@ -0,0 +1,17 @@
+//===---------------------- Unittests for errno --------------------------===//
+//
+// 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/errno/llvmlibc_errno.h"
+
+#include "gtest/gtest.h"
+
+TEST(ErrnoTest, Smoke) {
+  int test_val = 123;
+  llvmlibc_errno = test_val;
+  ASSERT_EQ(test_val, llvmlibc_errno);
+}
diff --git a/libc/src/errno/llvmlibc_errno.h b/libc/src/errno/llvmlibc_errno.h
new file mode 100644
--- /dev/null
+++ b/libc/src/errno/llvmlibc_errno.h
@@ -0,0 +1,22 @@
+//===------------------ Implementation header for errno -------------------===//
+//
+// 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_ERRNO_LLVMLIBC_ERRNO_H
+#define LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H
+
+// Internal code should use this macro and not use the errno macro from the
+// public header.
+#define llvmlibc_errno *__llvm_libc::__errno_location()
+
+namespace __llvm_libc {
+
+int *__errno_location();
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_ERRNO_LLVMLIBC_ERRNO_H