In the military, an adjutant is a regimental staff officer, who assists the colonel, or commanding officer of a garrison or regiment, in the details of regimental and garrison duty. In United States Army squadrons, the adjutant is often the officer-in-charge (OIC) of the administrative platoon.

In the British Army, the Adjutant (Adj; sometimes actually referred to as this) is usually a senior captain (sometimes a major). As the colonel's personal staff officer, he was once in charge of all the organisation, administration and discipline for a battalion or regiment, although now the bulk of administrative work is carried out by the Regimental Administrative Officer (RAO). Until the 1970s the adjutant was also the regimental operations officer, although this job is now filled by a separate officer. Unlike the RAO (who is an officer of the Adjutant General's Corps), the adjutant is a member of the corps or regiment of which their unit is a part. The adjutant's job is not solely a 'backroom' one, since he usually accompanies the colonel - Captain David Wood, the adjutant of 2 Para, was killed in action at the Battle of Goose Green, for example.

There is a bugle call announcing the adjutant that is still used in military ceremonies today.

An adjutant general is the principal staff officer of an army, through whom the commanding general receives communications and issues military orders. In the United States Army he is brigadier general.

Among the Jesuits, the adjutant general is one of a select number of fathers, who resided with the general of the order, each of whom had a province or country assigned to his care.

An adjutant is a species of very large stork (Ciconia argala ), a native of India, also called the gigantic crane, and by the native name argala, which feeds on carrion and offal, and is useful in this way, as storks are. It is noted for its serpent-destroying habits.