diff --git a/NEWS b/NEWS
index 86e05fb023..8e02dbd0f7 100644
--- a/NEWS
+++ b/NEWS
@@ -28,6 +28,12 @@ Major new features:
   The 32-bit RISC-V port requires at least Linux 5.4, GCC 7.1 and binutils
   2.28.
 
+* A new fortification level _FORTIFY_SOURCE=3 is available.  At this level,
+  glibc may use additional checks that may have an additional performance
+  overhead.  At present these checks are available only on LLVM 9 and later.
+  The latest GCC available at this time (10.2) does not support this level of
+  fortification.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The mallinfo function is marked deprecated.  Callers should call
diff --git a/include/features.h b/include/features.h
index 540230b90b..066eb0eecd 100644
--- a/include/features.h
+++ b/include/features.h
@@ -397,6 +397,11 @@
 #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
 # elif !__GNUC_PREREQ (4, 1)
 #  warning _FORTIFY_SOURCE requires GCC 4.1 or later
+# elif _FORTIFY_SOURCE > 2 && __glibc_clang_prereq (9, 0)
+#  if _FORTIFY_SOURCE > 3
+#   warning _FORTIFY_SOURCE > 3 is treated like 3 on this platform
+#  endif
+#  define __USE_FORTIFY_LEVEL 3
 # elif _FORTIFY_SOURCE > 1
 #  if _FORTIFY_SOURCE > 2
 #   warning _FORTIFY_SOURCE > 2 is treated like 2 on this platform
diff --git a/manual/creature.texi b/manual/creature.texi
index be5050468b..31208ccb2b 100644
--- a/manual/creature.texi
+++ b/manual/creature.texi
@@ -254,7 +254,8 @@ included.
 @standards{GNU, (none)}
 If this macro is defined to @math{1}, security hardening is added to
 various library functions.  If defined to @math{2}, even stricter
-checks are applied.
+checks are applied. If defined to @math{3}, @theglibc{} may also use
+checks that may have an additional performance overhead.
 @end defvr
 
 @defvr Macro _REENTRANT
diff --git a/misc/sys/cdefs.h b/misc/sys/cdefs.h
index a06f1cfd91..5fb6e309be 100644
--- a/misc/sys/cdefs.h
+++ b/misc/sys/cdefs.h
@@ -127,6 +127,15 @@
 #define __bos(ptr) __builtin_object_size (ptr, __USE_FORTIFY_LEVEL > 1)
 #define __bos0(ptr) __builtin_object_size (ptr, 0)
 
+/* Use __builtin_dynamic_object_size at _FORTIFY_SOURCE=3 when available.  */
+#if __USE_FORTIFY_LEVEL == 3 && __glibc_clang_prereq (9, 0)
+# define __glibc_objsize0(__o) __builtin_dynamic_object_size (__o, 0)
+# define __glibc_objsize(__o) __builtin_dynamic_object_size (__o, 1)
+#else
+# define __glibc_objsize0(__o) __bos0 (__o)
+# define __glibc_objsize(__o) __bos (__o)
+#endif
+
 #if __GNUC_PREREQ (4,3)
 # define __warnattr(msg) __attribute__((__warning__ (msg)))
 # define __errordecl(name, msg) \