A lieutenant (UK: lef-TEN-ənt, US: loo-TEN-ənt; abbreviated Lt., Lt, LT, Lieut and similar) is a commissioned officer rank in the armed forces of many nations.

The meaning of lieutenant differs in different militaries (see comparative military ranks), but it is often subdivided into senior (first lieutenant) and junior (second lieutenant and even third 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.

Lieutenant may also appear as part of a title used in various other organisations with a codified command structure. It often designates someone who is "second-in-command", and as such, may precede the name of the rank directly above it. For example, a "lieutenant master" is likely to be second-in-command to the "master" in an organisation using both ranks.

Political uses include lieutenant governor in various governments, such as the viceregal representatives of the Crown in Canadian provinces. In the United Kingdom, a lord lieutenant is the sovereign's representative in a county or lieutenancy area, while a deputy lieutenant is one of the lord lieutenant's deputies.