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
@@ -1,4 +1,7 @@
 set(TARGET_LIBC_ENTRYPOINTS
+    # ctype.h entrypoints
+    libc.src.ctype.isalpha
+    
     # errno.h entrypoints
     libc.src.errno.__errno_location
 
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
@@ -86,6 +86,14 @@
   ];
 }
 
+def CTypeAPI : PublicAPI<"ctype.h"> {
+  let TypeDeclarations = [];
+  
+  let Functions = [
+    "isalpha",
+  ];
+}
+
 def MathErrHandlingMacro : MacroDef<"math_errhandling"> {
   let Defn = [{
     #ifndef math_errhandling
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
@@ -1,6 +1,9 @@
 set(TARGET_LIBC_ENTRYPOINTS
     # assert.h entrypoints
     libc.src.assert.__assert_fail
+    
+    # ctype.h entrypoints
+    libc.src.ctype.isalpha
 
     # errno.h entrypoints
     libc.src.errno.__errno_location
diff --git a/libc/include/CMakeLists.txt b/libc/include/CMakeLists.txt
--- a/libc/include/CMakeLists.txt
+++ b/libc/include/CMakeLists.txt
@@ -17,10 +17,10 @@
     __llvm-libc-stdc-types.h
 )
 
-add_header(
+add_gen_header(
   ctype
-  HDR
-   ctype.h
+  DEF_FILE ctype.h.def
+  GEN_HDR ctype.h
   DEPENDS
     .llvm_libc_common_h
 )
diff --git a/libc/include/ctype.h.def b/libc/include/ctype.h.def
new file mode 100644
--- /dev/null
+++ b/libc/include/ctype.h.def
@@ -0,0 +1,16 @@
+//===-- C standard library header ctype.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_CTYPE_H
+#define LLVM_LIBC_CTYPE_H
+
+#include <__llvm-libc-common.h>
+
+%%public_api()
+
+#endif // LLVM_LIBC_CTYPE_H
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
--- a/libc/spec/stdc.td
+++ b/libc/spec/stdc.td
@@ -39,7 +39,21 @@
       [], // Enumerations
       []
   >;
-
+  
+  HeaderSpec CType = HeaderSpec<
+      "ctype.h",
+      [], // Macros
+      [], // Types
+      [], // Enumerations
+      [
+          FunctionSpec<
+              "isalpha",
+              RetValSpec<IntType>,
+              [ArgSpec<IntType>]
+          >,
+      ]
+  >;
+     
   HeaderSpec String = HeaderSpec<
       "string.h",
       [
@@ -386,6 +400,7 @@
 
   let Headers = [
     Assert,
+    CType,
     Errno,
     Math,
     String,
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(assert)
+add_subdirectory(ctype)
 add_subdirectory(errno)
 add_subdirectory(math)
 add_subdirectory(signal)
diff --git a/libc/src/ctype/CMakeLists.txt b/libc/src/ctype/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/libc/src/ctype/CMakeLists.txt
@@ -0,0 +1,7 @@
+add_entrypoint_object(
+  isalpha
+  SRCS
+    isalpha.cpp
+  HDRS
+    isalpha.h
+)
diff --git a/libc/src/ctype/isalpha.h b/libc/src/ctype/isalpha.h
new file mode 100644
--- /dev/null
+++ b/libc/src/ctype/isalpha.h
@@ -0,0 +1,18 @@
+//===-- Implementation header for isalpha -------------------------*-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_CTYPE_ISALPHA_H
+#define LLVM_LIBC_SRC_CTYPE_ISALPHA_H
+
+namespace __llvm_libc {
+
+int isalpha(int c);
+
+} // namespace __llvm_libc
+
+#endif //  LLVM_LIBC_SRC_CTYPE_ISALPHA_H
diff --git a/libc/src/ctype/isalpha.cpp b/libc/src/ctype/isalpha.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/ctype/isalpha.cpp
@@ -0,0 +1,22 @@
+//===-- Implementation of isalpha------------------------------------------===//
+//
+// 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/ctype/isalpha.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+// TODO: Currently restricted to default locale.
+// These should be extended using locale information.
+int LLVM_LIBC_ENTRYPOINT(isalpha)(int c) {
+  const unsigned ch = c;
+  return (ch | 32) - 'a' < 26;
+}
+
+} // namespace __llvm_libc
diff --git a/libc/test/src/CMakeLists.txt b/libc/test/src/CMakeLists.txt
--- a/libc/test/src/CMakeLists.txt
+++ b/libc/test/src/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_subdirectory(assert)
+add_subdirectory(ctype)
 add_subdirectory(errno)
 add_subdirectory(math)
 add_subdirectory(signal)
@@ -91,4 +92,4 @@
 target_link_libraries(libc-integration-test
   PRIVATE
   ${library_files}
-)
\ No newline at end of file
+)
diff --git a/libc/test/src/ctype/CMakeLists.txt b/libc/test/src/ctype/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/libc/test/src/ctype/CMakeLists.txt
@@ -0,0 +1,11 @@
+add_libc_testsuite(libc_ctype_unittests)
+
+add_libc_unittest(
+  isalpha
+  SUITE
+    libc_ctype_unittests
+  SRCS
+    isalpha_test.cpp
+  DEPENDS
+    libc.src.ctype.isalpha
+)
diff --git a/libc/test/src/ctype/isalpha_test.cpp b/libc/test/src/ctype/isalpha_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/ctype/isalpha_test.cpp
@@ -0,0 +1,26 @@
+//===-- Unittests for isalpha----------------------------------------------===//
+//
+// 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/ctype/isalpha.h"
+#include "utils/UnitTest/Test.h"
+
+// Helper function that makes a call to isalpha a bit cleaner
+// for use with testing utilities, since it explicitly requires
+// a boolean value for EXPECT_TRUE and EXPECT_FALSE.
+bool call_isalpha(int c) { return __llvm_libc::isalpha(c); }
+
+TEST(IsAlpha, DefaultLocale) {
+  // Loops through all characters, verifying that letters return true
+  // and everything else returns false.
+  for (int ch = 0; ch < 255; ++ch) {
+    if (('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z'))
+      EXPECT_TRUE(call_isalpha(ch));
+    else
+      EXPECT_FALSE(call_isalpha(ch));
+  }
+}