The Full Form of SGM is Sergeant Major.

Sergeant major is a senior non-commissioned rank or appointment in many militaries around the world. In Commonwealth countries, the various degrees of sergeant major are appointments held by warrant officers. In the United States, there are also various grades of sergeant major, all of the same pay grade of E-9; however, the Sergeant Major of the Army and the Sergeant Major of the Marine Corps, as their respective service’s Senior Enlisted Advisor, receive a special rate of basic pay that is higher than all other sergeants major.