The senior military assistant to the secretary of defense (SMA SecDef)[1] is the highest-ranking commissioned officer in the Office of the Secretary of Defense (OSD).

The SMA directly reports to the secretary of defense, performing administrative work in the Secretary's name and handles relations between the Secretary and other pertinent government bodies, including the rest of the OSD, Congress and the Joint Chiefs of Staff.

[1] As a joint duty assignment, the SMA is one of 310 officer positions granted exemption from statutory limits on general officer numbers under 10 U.S.C. § 526.

The SMA holds the rank of lieutenant general or vice admiral.

They are nominated by the president and must be confirmed by the Senate.