A lieutenant (abbreviated Lt., LT, LTA, Lieut., LEUT and LEUT.) is a junior commissioned officer in many nations' armed forces, fire service or police.

The meaning of lieutenant differs in different military formations (see comparative military ranks), but is often subdivided into senior (first lieutenant) and junior (second lieutenant) ranks. In navies it is often equivalent to the army rank of captain; it may also indicate a particular post rather than a rank. The rank is also used in fire services, emergency medical services, security services and police forces.