diff --git a/libcxx/docs/Hardening.rst b/libcxx/docs/Hardening.rst
index 42aacfdcfb41a..4002f40e1dad3 100644
--- a/libcxx/docs/Hardening.rst
+++ b/libcxx/docs/Hardening.rst
@@ -458,7 +458,7 @@ Hardened containers status
       - Partial
       - N/A
     * - ``bitset``
-      - ❌
+      - ✅
       - N/A
 
 Note: for ``vector`` and ``string``, the iterator does not check for
diff --git a/libcxx/include/bitset b/libcxx/include/bitset
index e2dc7b459f1c4..df9fdfe6dd9ef 100644
--- a/libcxx/include/bitset
+++ b/libcxx/include/bitset
@@ -132,6 +132,7 @@ template <size_t N> struct hash<std::bitset<N>>;
 #  include <__algorithm/fill.h>
 #  include <__algorithm/fill_n.h>
 #  include <__algorithm/find.h>
+#  include <__assert>
 #  include <__bit_reference>
 #  include <__config>
 #  include <__functional/hash.h>
@@ -682,13 +683,18 @@ public:
 
   // element access:
 #  ifdef _LIBCPP_ABI_BITSET_VECTOR_BOOL_CONST_SUBSCRIPT_RETURN_BOOL
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool operator[](size_t __p) const { return __base::__make_ref(__p); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool operator[](size_t __p) const {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__p < _Size, "bitset::operator[] index out of bounds");
+    return __base::__make_ref(__p);
+  }
 #  else
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR const_reference operator[](size_t __p) const {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__p < _Size, "bitset::operator[] index out of bounds");
     return __base::__make_ref(__p);
   }
 #  endif
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 reference operator[](size_t __p) {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__p < _Size, "bitset::operator[] index out of bounds");
     return __base::__make_ref(__p);
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 unsigned long to_ulong() const;
diff --git a/libcxx/include/valarray b/libcxx/include/valarray
index af916096b5ef9..e86367dac929d 100644
--- a/libcxx/include/valarray
+++ b/libcxx/include/valarray
@@ -821,9 +821,15 @@ public:
   _LIBCPP_HIDE_FROM_ABI valarray& operator=(const __val_expr<_ValExpr>& __v);
 
   // element access:
-  _LIBCPP_HIDE_FROM_ABI const value_type& operator[](size_t __i) const { return __begin_[__i]; }
+  _LIBCPP_HIDE_FROM_ABI const value_type& operator[](size_t __i) const {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__i < size(), "valarray::operator[] index out of bounds");
+    return __begin_[__i];
+  }
 
-  _LIBCPP_HIDE_FROM_ABI value_type& operator[](size_t __i) { return __begin_[__i]; }
+  _LIBCPP_HIDE_FROM_ABI value_type& operator[](size_t __i) {
+    _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__i < size(), "valarray::operator[] index out of bounds");
+    return __begin_[__i];
+  }
 
   // subset operations:
   _LIBCPP_HIDE_FROM_ABI __val_expr<__slice_expr<const valarray&> > operator[](slice __s) const;
diff --git a/libcxx/test/libcxx/numerics/numarray/assert.pass.cpp b/libcxx/test/libcxx/numerics/numarray/assert.pass.cpp
new file mode 100644
index 0000000000000..2bdf52340abfc
--- /dev/null
+++ b/libcxx/test/libcxx/numerics/numarray/assert.pass.cpp
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <valarray>
+
+// Test hardening assertions for std::valarray.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: libcpp-hardening-mode=none
+// UNSUPPORTED: c++03
+// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
+
+#include <valarray>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  { // Empty valarray
+    std::valarray<int> c;
+    const auto& const_c = c;
+    TEST_LIBCPP_ASSERT_FAILURE(c[0], "valarray::operator[] index out of bounds");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c[0], "valarray::operator[] index out of bounds");
+    TEST_LIBCPP_ASSERT_FAILURE(c[42], "valarray::operator[] index out of bounds");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c[42], "valarray::operator[] index out of bounds");
+  }
+
+  { // Non-empty valarray
+    std::valarray<int> c(4);
+    const auto& const_c = c;
+    (void)c[3]; // Check that there's no assertion on valid access.
+    TEST_LIBCPP_ASSERT_FAILURE(c[4], "valarray::operator[] index out of bounds");
+    (void)const_c[3]; // Check that there's no assertion on valid access.
+    TEST_LIBCPP_ASSERT_FAILURE(const_c[4], "valarray::operator[] index out of bounds");
+  }
+
+  return 0;
+}
diff --git a/libcxx/test/libcxx/utilities/template.bitset/assert.pass.cpp b/libcxx/test/libcxx/utilities/template.bitset/assert.pass.cpp
new file mode 100644
index 0000000000000..4019bdf1318eb
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/template.bitset/assert.pass.cpp
@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <bitset>
+
+// Test hardening assertions for std::bitset.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: libcpp-hardening-mode=none
+// UNSUPPORTED: c++03
+// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
+
+#include <bitset>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  { // Empty bitset
+    std::bitset<0> c;
+    const auto& const_c = c;
+    TEST_LIBCPP_ASSERT_FAILURE(c[0], "bitset::operator[] index out of bounds");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c[0], "bitset::operator[] index out of bounds");
+    TEST_LIBCPP_ASSERT_FAILURE(c[42], "bitset::operator[] index out of bounds");
+    TEST_LIBCPP_ASSERT_FAILURE(const_c[42], "bitset::operator[] index out of bounds");
+  }
+
+  { // Non-empty bitset
+    std::bitset<4> c(42);
+    const auto& const_c = c;
+    (void)c[3]; // Check that there's no assertion on valid access.
+    TEST_LIBCPP_ASSERT_FAILURE(c[4], "bitset::operator[] index out of bounds");
+    (void)const_c[3]; // Check that there's no assertion on valid access.
+    TEST_LIBCPP_ASSERT_FAILURE(const_c[4], "bitset::operator[] index out of bounds");
+  }
+
+  return 0;
+}
diff --git a/libcxx/test/std/utilities/template.bitset/bitset.members/op_and_eq.pass.cpp b/libcxx/test/std/utilities/template.bitset/bitset.members/op_and_eq.pass.cpp
index e8ab264f4bab6..d87fd91b0356c 100644
--- a/libcxx/test/std/utilities/template.bitset/bitset.members/op_and_eq.pass.cpp
+++ b/libcxx/test/std/utilities/template.bitset/bitset.members/op_and_eq.pass.cpp
@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-steps): -fconstexpr-steps=2000000
+
 // bitset<N>& operator&=(const bitset<N>& rhs); // constexpr since C++23
 
 #include <bitset>