diff --git a/MdePkg/Include/IndustryStandard/pci22.h b/MdePkg/Include/IndustryStandard/pci22.h
index f150fbc1b0..581744ff55 100644
--- a/MdePkg/Include/IndustryStandard/pci22.h
+++ b/MdePkg/Include/IndustryStandard/pci22.h
@@ -319,6 +319,11 @@ typedef struct {
 #define PCI_BRIDGE_SECONDARY_BUS_REGISTER_OFFSET    0x19
 #define PCI_BRIDGE_SUBORDINATE_BUS_REGISTER_OFFSET  0x1a
 
+//
+// Interrupt Line "Unknown" or "No connection" value defined for x86 based system
+//
+#define PCI_INT_LINE_UNKNOWN                        0xFF               
+
 typedef union {
   struct {
     UINT32  Reg : 8;