First sergeant is typically a senior non-commissioned officer rank, used in many countries.

First sergeant is a specialist in the Singapore Armed Forces. First sergeants are the most senior of the junior specialists, ranking above second sergeants, and below Staff Sergeants. The rank insignia for a First Sergeant features the three chevrons pointing down shared by all specialists, and two chevrons pointing up.

In combat units, First Sergeants are very often platoon sergeants or given the responsibility for independently operating detachments of support weapons. They are often given instructional billets as well in training schools. First sergeants normally answer to the company sergeant major, assisting the latter in the mentorship, guidance and command of the more junior specialists (Third and Second Sergeants) who are section commanders.

In the United States, a first sergeant generally serves as the senior enlisted advisor (SEA) of a unit, such as a company, battery, or troop, or a USAF squadron or higher level unit. (USA and USMC squadrons and battalions, as well as all higher-level units, have a Command Sergeant Major [USA] or Sergeant Major [USMC] as the SEA.) While the specifics of the title may differ between the United States Army, Marine Corps, and Air Force, all first sergeants can be identified by the presence of a lozenge-shaped (colloquially "diamond") figure on their rank insignia.

The rank of "first sergeant" has existed in the American Army since 1781, when a fifth sergeant was added to the table of organization for Continental Army infantry regiments. Previously, under the tables of organization approved by the Continental Congress in 1776 and 1779, there were four and three sergeants, respectively, authorized in each company. The sergeants were numbered in order of seniority, and the "first sergeant" was simply the senior sergeant in the company, but not a separate rank. After the 1781 Battle of Green Spring, Ebenezer Denny called the company first sergeant "the most important officer."

In 1833, first sergeant and orderly sergeant became separate pay grades, ranking below sergeants major and quartermaster sergeants, but above sergeants. In 1851, first sergeant was combined with the separate rank of orderly sergeant.

In the United States Army, since 1958, the rank of first sergeant (abbreviated 1SG) is considered a temporary rank in E-8 pay grade, ranking above sergeant first class (SFC), and below sergeant major (SGM) or command sergeant major (CSM). While first sergeant is equal in paygrade to master sergeant (MSG), the two ranks have different responsibilities. Both ranks are identical as indicated by three chevrons (standard sergeant insignia) above three inverted arcs ("rockers"), an arrangement commonly referred to as "three up and three down", though the first sergeant has the pierced lozenge "diamond" in the middle. A first sergeant is generally senior to a master sergeant in leadership matters, though a master sergeant may have more general military authority such as when in charge of a military police (MP) section.

Master sergeants are laterally appointed to first sergeant upon selection by senior Department of the Army leadership; qualified sergeants first class are promoted, depending on available billets and opportunities. A promotable sergeant first class, or a master sergeant, may be selected for promotion to, or appointment as, a first sergeant, but will continue to wear the initial rank until successful completion of first sergeant leadership school, when the 1SG insignia of rank is authorized ("getting one's diamond"). Upon reassignment to a non–first sergeant billet, the soldier reverts to rank of master sergeant, unless promoted to the E-9 rank of SGM or CSM. First sergeants can retire at that rank, if they so choose, as long as they have completed first sergeant school and served satisfactorily in rank.