A flag officer is a commissioned officer in a nation's armed forces senior enough to be entitled to fly a flag to mark the position from which that officer exercises command.

Different countries use the term "flag officer" in different ways:

The generic title of flag officer is used in many modern navies and coast guards to denote those who hold the rank of rear admiral or its equivalent and above, also called "flag ranks". In some navies, this also includes the rank of commodore. Flag officer corresponds to the generic terms general officer, used by land and some air forces to describe all grades of generals, and air officer, used by other air forces to describe all grades of air marshals and air commodores.

A flag officer sometimes is a junior officer, called a flag lieutenant or flag adjutant, attached as a personal adjutant or aide-de-camp.

In the Canadian Armed Forces, a flag officer (French: officier général, "general officer") is an admiral, vice admiral, rear admiral, or commodore, the naval equivalent of a general officer of the army or air force. It is a somewhat counterintuitive usage of the term, as only flag officers in command of commands or formations actually have their own flags (technically a commodore has only a broad pennant, not a flag), and army and air force generals in command of commands or formations also have their own flags, but are not called flag officers. Base commanders, usually full colonels, have a pennant that flies from the mast or flagpole on the base, when resident, or on vehicles that carry them.

A flag officer's rank is denoted by a wide strip of gold braid on the cuff of the service dress tunic, one to four gold maple leaves over a crossed sword and baton, all beneath a royal crown, on epaulettes and shoulder boards; and two rows of gold oak leaves on the peak of the service cap. Since the unification of the Canadian Forces in 1968, a flag officer's dress tunic had a single broad stripe on the sleeve and epaulettes.

In May 2010 the naval uniform dark dress tunic was adjusted—exterior epaulettes were removed, reverting to the sleeve ring and executive curl-rank insignia used by most navies. commodores' uniforms display a broad stripe, and each succeeding rank receives an additional sleeve ring. There are no epaulettes on the exterior of the tunic, but they are still worn on the uniform shirt underneath.

In the Indian Armed Forces, it is applied to brigadiers, major generals, lieutenant generals and generals in the Army; commodores, rear admirals, vice admirals and admirals in the Navy; and air commodores, air vice marshals, air marshals and air chief marshals in the Air Force. Each of these flag officers are designated with a specific flag. India's honorary ranks (five star ranks) are field marshal in the Army, Marshal of the Indian Air Force in the Air Force and admiral of the fleet in the Navy. A similar equivalence is applied to senior police officers of rank Deputy Inspector General (DIG), Inspector General (IG), Additional Director General (ADG) and Director General (DG).