Warrant officer (WO) is a rank or category of ranks in the armed forces of many countries. Depending on the service or historical context, some WOs are classified as the most junior of the commissioned ranks, the most senior of the NCO ranks or in a separate category of their own. However, warrant officer ranks are always senior to non-commissioned officer (NCO) ranks and subordinate to commissioned officer ranks. WO 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 the requirements of 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).