Flight lieutenant (Flt Lt in the RAF and IAF; FLTLT in the RAAF and RNZAF, formerly F/L in the RCAF) is a junior commissioned rank in the Royal Air Force and the air forces of many Commonwealth countries. It ranks above flying officer and immediately below squadron leader. The name of the rank is the complete phrase; it is never shortened to "lieutenant". In RAF informal usage, a flight lieutenant is sometimes referred to as a "flight lieuy".

It has a NATO ranking code of OF-2, and is equivalent to a lieutenant in the Royal Navy or a captain in the British Army or the Royal Marines.

The equivalent rank in the Women's Auxiliary Air Force (WAAF), Women's Royal Air Force (WRAF) (until 1968) and Princess Mary's Royal Air Force Nursing Service (PMRAFNS) (until 1980) was flight officer.

On 1 April 1918, the newly created RAF adopted its officer rank titles from the British Army, with Royal Naval Air Service lieutenants (entitled flight lieutenants and flight commanders) and Royal Flying Corps captains becoming captains in the RAF. In response to the proposal that the RAF should use its own rank titles, it was suggested that the RAF might use the Royal Navy's officer ranks, with the word "air" inserted before the naval rank title. For example, the current rank of flight lieutenant would have been "air lieutenant". Although the Admiralty objected to this simple modification of their rank titles, it was agreed that the RAF might base many of its officer rank titles on navy officer ranks with differing pre-modifying terms. It was also suggested that RAF captains might be entitled flight-leaders. However, the rank title flight lieutenant was chosen as flights were typically commanded by RAF captains and the term flight lieutenant had been used in the Royal Naval Air Service. The rank of flight lieutenant has been used continuously since 1 August 1919.