diff --git a/libc/test/src/stdlib/atexit_test.cpp b/libc/test/src/stdlib/atexit_test.cpp
@@ -0,0 +1,94 @@
+//===-- Unittests for atexit ----------------------------------------------===//
+//
+// 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/__support/CPP/Array.h"
+#include "src/__support/CPP/Utility.h"
+#include "src/stdlib/atexit.h"
+#include "src/stdlib/exit.h"
+#include "utils/UnitTest/Test.h"
+
+static int a;
+TEST(LlvmLibcAtExit, Basic) {
+  // In case tests ever run multiple times.
+  a = 0;
+
+  auto test = [] {
+    int status = __llvm_libc::atexit(+[] {
+      if (a != 1)
+        __builtin_trap();
+    });
+    status |= __llvm_libc::atexit(+[] { a++; });
+    if (status)
+      __builtin_trap();
+
+    __llvm_libc::exit(0);
+  };
+  EXPECT_EXITS(test, 0);
+}
+
+TEST(LlvmLibcAtExit, AtExitCallsSysExit) {
+  auto test = [] {
+    __llvm_libc::atexit(+[] { _Exit(1); });
+    __llvm_libc::exit(0);
+  };
+  EXPECT_EXITS(test, 1);
+}
+
+static int size;
+static __llvm_libc::cpp::Array<int, 256> arr;
+
+template <int... Ts>
+void register_atexit_handlers(__llvm_libc::cpp::IntegerSequence<int, Ts...>) {
+  (__llvm_libc::atexit(+[] { arr[size++] = Ts; }), ...);
+}
+
+template <int count> constexpr auto getTest() {
+  return [] {
+    __llvm_libc::atexit(+[] {
+      if (size != count)
+        __builtin_trap();
+      for (int i = 0; i < count; i++)
+        if (arr[i] != count - 1 - i)
+          __builtin_trap();
+    });
+    register_atexit_handlers(
+        __llvm_libc::cpp::MakeIntegerSequence<int, count>{});
+    __llvm_libc::exit(0);
+  };
+}
+
+TEST(LlvmLibcAtExit, ReverseOrder) {
+  // In case tests ever run multiple times.
+  size = 0;
+
+  auto test = getTest<32>();
+  EXPECT_EXITS(test, 0);
+}
+
+TEST(LlvmLibcAtExit, Many) {
+  // In case tests ever run multiple times.
+  size = 0;
+
+  auto test = getTest<256>();
+  EXPECT_EXITS(test, 0);
+}
+
+// POSIX doesn't specify if an atexit handler can call atexit, it only says it
+// is undefined for a handler to call exit(3). The current implementation will
+// end up invoking the newly registered function, although glibc does, other
+// libc's do not. This just tests that we don't deadlock when an exit handler
+// calls atexit.
+TEST(LlvmLibcAtExit, HandlerCallsAtExit) {
+  auto test = [] {
+    __llvm_libc::atexit(+[] {
+      __llvm_libc::atexit(+[] { __builtin_trap(); });
+      __llvm_libc::exit(0);
+    });
+  };
+  EXPECT_EXITS(test, 0);
+}