First officer may refer to:
Transportation Chief mate, the second-in-command (usually) of a merchant ship First officer (aviation) or "co-pilot", the second-in-command of a civil aircraft
Military Executive officer (military), the second-in-command of a naval vessel First officer, the most senior civil service position in the United States Foreign Service First officer, a rank in the Air Transport Auxiliary corresponding to flight lieutenant in the Royal Air Force First officer, a rank in the Women's Royal Naval Service corresponding to lieutenant-commander in the Royal Navy