In the United States Armed Forces, a line officer or officer of the line is a U.S. Navy or U.S. Marine Corps commissioned officer or warrant officer who exercises general command authority and is eligible for operational command positions, as opposed to officers who normally exercise command authority only within a Navy Staff Corps. The term line officer is also used by the U.S. Air Force and U.S. Coast Guard to indicate that an officer is eligible for command of operational, viz., tactical or combat units. The term is not generally used by officers of the U.S. Army – the roughly corresponding Army terms are basic branch (e.g, Infantry) and special branch (e.g., Medical Corps) qualified officers, although the concepts are not entirely synonymous, as some Army special branch officers (e.g., Judge Advocate General's Corps) are eligible to hold command outside their branch specialty.

Officers who are not line officers are those whose primary duties are generally in non-combat specialties including (depending upon the service) attorneys, chaplains, civil engineers, health services professionals, and logistics and financial management specialists. A line officer may hold authority over a non-line officer of higher rank by the nature of their assignment or appointment/succession to command, but is otherwise expected to observe normal customs and courtesies outside that role.

See explanation of staff and line.