The sergeant major of the Marine Corps is a billet, as well as a unique enlisted grade of rank, and is designated a special paygrade above E-9. The position also has a unique non-commissioned grade of rank insignia, in the United States Marine Corps. The holder of this rank and position is the most senior enlisted marine in the Marine Corps, unless an enlisted marine is serving as the senior enlisted advisor to the chairman, which is the case as of November 3, 2023.

The senior enlisted advisor to the chairman of the Joint Chiefs of Staff (SEAC) is the most senior non-commissioned officer (NCO) position overall in the United States Armed Forces. The SEAC is appointed by the chairman of the Joint Chiefs of Staff to serve as a spokesperson to address the issues of enlisted personnel to the highest positions in the Department of Defense. As such, the SEAC is the primary enlisted advisor to the chairman, and serves at the pleasure of the secretary of defense. The SEAC's exact duties vary, depending on the chairman, though the SEAC generally devotes much of their time traveling throughout the Department of Defense, to observe training and communicating to service members and their families. The SEAC's normal term of assignment runs concurrently with the chairman, but an incumbent may be reappointed to serve longer. The first member to hold this post was William Gainey. The current SEAC is Troy E. Black, USMC who assumed the duties on 3 November 2023.