Warrant officer (WO) is a rank or category of ranks in the armed forces of many countries. Depending on the country, service, or historical context, warrant officers are sometimes classified as the most junior of the commissioned officer ranks, the most senior of the non-commissioned officer (NCO) ranks, or in a separate category of their own. Warrant officer ranks are especially prominent in the militaries of Commonwealth nations and the United States.

The name of the rank originated in medieval England. It was first used during the 13th century, in the Royal Navy, where warrant officers achieved the designation by virtue of their accrued experience or seniority, and technically held the rank by a warrant, rather than by a formal commission (as in the case of a commissioned officer). Nevertheless, WOs in the British services have traditionally been considered and treated as distinct from non-commissioned officers, as such (even though neither group has, technically, held a commission).

Warrant officers in the United States are classified in rank category "W", which is distinct from "O" (commissioned officers) and "E" (enlisted personnel). However, chief warrant officers are officially commissioned, on the same basis as commissioned officers, and take the same oath. US WOs are usually experts in a particular technical field, with long service as enlisted personnel; in some cases, however, direct entrants may become WOs—for example, individuals completing helicopter pilot training in the US Army Aviation Branch become flight warrant officers immediately.

In Commonwealth countries, warrant officers have usually been included alongside NCOs and enlisted personnel in a category called other ranks (ORs), which is equivalent to the US "E" category (i.e. there is no separate "W" category in these particular services). In Commonwealth services, warrant officers rank between chief petty officer and sub-lieutenant in the navy, between staff sergeant and second lieutenant in the army, and between flight sergeant and pilot officer in the air force.