diff --git a/openair2/COMMON/ngap_messages_types.h b/openair2/COMMON/ngap_messages_types.h
index 29ea48294a..faeb11d377 100644
--- a/openair2/COMMON/ngap_messages_types.h
+++ b/openair2/COMMON/ngap_messages_types.h
@@ -409,6 +409,19 @@ typedef enum ngap_Cause_radio_network_e {
   NGAP_CAUSE_RADIO_NETWORK_MULTIPLE_LOCATION_REPORTING_REFERENCE_ID_INSTANCES
 } ngap_Cause_radio_network_t;
 
+/**
+ * NGAP protocol cause values as per 9.3.1.2 `Cause` section in 3GPP TS 38.413.
+ */
+typedef enum ngap_cause_protocol_e {
+  NGAP_CAUSE_PROTOCOL_TRANSFER_SYNTAX_ERROR,
+  NGAP_CAUSE_PROTOCOL_ABSTRACT_SYNTAX_ERROR_REJECT,
+  NGAP_CAUSE_PROTOCOL_ABSTRACT_SYNTAX_ERROR_IGNORE,
+  NGAP_CAUSE_PROTOCOL_MSG_NOT_COMPATIBLE_WITH_RECEIVER_STATE,
+  NGAP_CAUSE_PROTOCOL_SEMANTIC_ERROR,
+  NGAP_CAUSE_PROTOCOL_ABSTRACT_SYNTAX_ERROR_FCM,
+  NGAP_CAUSE_PROTOCOL_UNSPECIFIED
+} ngap_cause_protocol_t;
+
 typedef struct pdusession_failed_s {
   /* Unique pdusession_id for the UE. */
   uint8_t pdusession_id;