Flag Officer is both a historic naval rank and a modern day navy title. The title of "Flag Officer" is used in several 21st century navies to denote those who hold the rank of Rear Admiral (or its equivalent) and above. This corresponds to the term General Officer used by land forces to describe all grades of Generals.

In the 19th century, Flag Officer was an actual rank of the United States Navy and, during the American Civil War, also the Confederate Navy. The rank of Flag Officer was bestowed on senior Navy Captains who were assigned to lead a squadron of vessels in addition to command of thier own ship. The 19th century rank of Flag Officer was considered strictly temporary and became obsolete upon the creation and wide spread usage of the rank Commodore.