diff --git a/llvm/include/llvm/BinaryFormat/ELF.h b/llvm/include/llvm/BinaryFormat/ELF.h
index b84b4e84f..4e45ece44 100644
--- a/llvm/include/llvm/BinaryFormat/ELF.h
+++ b/llvm/include/llvm/BinaryFormat/ELF.h
@@ -326,6 +326,12 @@ enum {
   EM_CSKY = 252,          // C-SKY 32-bit processor
   EM_LOONGARCH = 258,     // LoongArch
   EM_MOS = 6502,          // MOS Technologies 65xx
+  EM_W65816 = 0xFF16,     // WDC 65C816 / Apple IIgs (vendor-private, see
+                          // docs/USAGE.md "ELF e_machine value" section).
+                          // Sits in the 0xFF00-0xFFFF experimental/vendor
+                          // range reserved by the ELF spec; the "16" suffix
+                          // is mnemonic for "65816".  Picked here because
+                          // 65816 itself (0x10118) does not fit Elf32_Half.
 };
 
 // Object file classes.
