diff --git a/docs/markdown/snippets/cpp-stldebug.md b/docs/markdown/snippets/cpp-stldebug.md
new file mode 100644
index 000000000000..ba0473608c8a
--- /dev/null
+++ b/docs/markdown/snippets/cpp-stldebug.md
@@ -0,0 +1,4 @@
+## `stldebug` gains Clang support
+
+For Clang, we now pass `-D_GLIBCXX_DEBUG=1` if `debugstl` is enabled, and
+we also pass `-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG`.
diff --git a/mesonbuild/compilers/cpp.py b/mesonbuild/compilers/cpp.py
index f6a03caf218b..998486bd4b80 100644
--- a/mesonbuild/compilers/cpp.py
+++ b/mesonbuild/compilers/cpp.py
@@ -242,6 +242,10 @@ def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = CPPCompiler.get_options(self)
         key = OptionKey('key', machine=self.for_machine, lang=self.language)
         opts.update({
+            key.evolve('debugstl'): coredata.UserBooleanOption(
+                'STL debug mode',
+                False,
+            ),
             key.evolve('eh'): coredata.UserComboOption(
                 'C++ exception handling type.',
                 ['none', 'default', 'a', 's', 'sc'],
@@ -277,6 +281,9 @@ def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]
 
         non_msvc_eh_options(options[key.evolve('eh')].value, args)
 
+        if options[key.evolve('debugstl')].value:
+            args.append('-D_GLIBCXX_DEBUG=1')
+
         if not options[key.evolve('rtti')].value:
             args.append('-fno-rtti')
 
@@ -649,6 +656,13 @@ def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]
             args.append('-fno-rtti')
         if options[key.evolve('debugstl')].value:
             args.append('-D_GLIBCXX_DEBUG=1')
+
+            # We can't do _LIBCPP_DEBUG because it's unreliable unless libc++ was built with it too:
+            # https://discourse.llvm.org/t/building-a-program-with-d-libcpp-debug-1-against-a-libc-that-is-not-itself-built-with-that-define/59176/3
+            # Note that unlike _GLIBCXX_DEBUG, _MODE_DEBUG doesn't break ABI. It's just slow.
+            if version_compare(self.version, '>=18'):
+                args.append('-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG')
+
         return args
 
     def get_option_link_args(self, options: 'KeyedOptionDictType') -> T.List[str]: