diff --git a/gcc/config/rs6000/rs6000-c.cc b/gcc/config/rs6000/rs6000-c.cc
index 15251efc209..d2e480ad7df 100644
--- a/gcc/config/rs6000/rs6000-c.cc
+++ b/gcc/config/rs6000/rs6000-c.cc
@@ -623,7 +623,11 @@ rs6000_cpu_cpp_builtins (cpp_reader *pfile)
   if (TARGET_FRSQRTES)
     builtin_define ("__RSQRTEF__");
   if (TARGET_FLOAT128_TYPE)
-    builtin_define ("__FLOAT128_TYPE__");
+      builtin_define ("__FLOAT128_TYPE__");
+  if (ibm128_float_type_node)
+    builtin_define ("__SIZEOF_IBM128__=16");
+  if (ieee128_float_type_node)
+    builtin_define ("__SIZEOF_FLOAT128__=16");
 #ifdef TARGET_LIBC_PROVIDES_HWCAP_IN_TCB
   builtin_define ("__BUILTIN_CPU_SUPPORTS__");
 #endif
diff --git a/gcc/testsuite/gcc.target/powerpc/pr99708.c b/gcc/testsuite/gcc.target/powerpc/pr99708.c
new file mode 100644
index 00000000000..d478f7bc4c0
--- /dev/null
+++ b/gcc/testsuite/gcc.target/powerpc/pr99708.c
@@ -0,0 +1,21 @@
+/* { dg-do run } */
+/* { require-effective-target ppc_float128_sw } */
+/* { dg-options "-O2 -mvsx -mfloat128" } */
+
+/*
+ * PR target/99708
+ *
+ * Verify that __SIZEOF_FLOAT128__ and __SIZEOF_IBM128__ are properly defined.
+ */
+
+#include <stdlib.h>
+
+int main (void)
+{
+  if (__SIZEOF_FLOAT128__ != sizeof (__float128)
+      || __SIZEOF_IBM128__ != sizeof (__ibm128))
+    abort ();
+
+  return 0;
+}
+