First lieutenant is a commissioned officer military rank in many armed forces; in some forces, it is an appointment.

The rank of lieutenant has different meanings in different military formations, but in most forces it is sub-divided into a senior (first lieutenant) and junior (second lieutenant) rank.

In navies, while certain rank insignia may carry the name lieutenant, the term may also be used to relate to a particular post or duty, rather than a rank.

In Indonesia, "first lieutenant" is known as Letnan Satu (Lettu), Indonesian National Armed Forces uses this rank across all three of its services. It is just above the rank of second lieutenant and just below the rank of captain.

In the Israel Defense Forces, the rank above second lieutenant is simply lieutenant (Segen). The rank of (קצין מקצועי אקדמאי (קמ"א (katsín miktsoí akademai or "kama"), a professional academic officer (that is, a medical, dental or veterinary officer, a justice officer or a religious officer), is equivalent to a professional officer of the second class in the reserve and equivalent to first lieutenant.^{[citation needed]}

In the British Army and Royal Marines, the rank above second lieutenant is simply lieutenant (pronounced lef-tenant), with no ordinal attached.

Before 1871, when the whole British Army switched to using the current rank of "lieutenant", the Royal Artillery, Royal Engineers and fusilier regiments used "first lieutenant" and "second lieutenant".

The first lieutenant (often abbreviated "1st Lt") in a Royal Navy ship is a post or appointment, rather than a rank.