A warrant officer (WO) is an officer in a military organisation who is designated an officer by a warrant, as distinguished from a commissioned officer who is designated an officer by a commission, and a non-commissioned officer who is designated an officer, often by virtue of seniority.

The rank was first used in the (then) English Royal Navy and is today used in most services in many countries, including the Commonwealth nations and the United States.

Outside the United States, warrant officers are included in the "Other Ranks" (OR) category, equivalent to the US "E" (Enlisted) category and rank between non-commissioned officers and commissioned officers. The warrant officers in Commonwealth navies rank between chief petty officer and sub-lieutenant, in Commonwealth air forces between flight sergeant and pilot officer, and in Commonwealth armies between staff sergeant and second-lieutenant.

Warrant officers in the United States are classified as officers and are in the "W" category (NATO "WO"); they are technical leaders and specialists. Chief warrant officers are commissioned by the President of the United States and take the same oath as regular commissioned officers. They may be technical experts with a long service as enlisted personnel, or direct entrants, notably for U.S. Army helicopter pilots.

In the United States Armed Forces, the ranks of warrant officer (grades W-1 to W-5) (NATO: WO1-WO5) are rated as officers above the senior-most enlisted ranks, including all officer candidates (OC) and warrant officer candidates (WOC) (both of whom hold enlisted grades as E-5 or above, depending upon their grade upon appointment as an OC or WOC); and cadets and midshipmen (who are considered "officers only in a qualified sense", holding a special grade above E-9 and below W-1), but subordinate to the officer grade of O-1 (NATO: OF-1). This application differs from the Commonwealth of Nations and other militaries, where warrant officers are the most senior of the other ranks (NATO: OR-8 and OR-9), equivalent to the US Armed Forces grades of E-8 and E-9.

Warrant officers are highly skilled, single-track specialty officers, and while the ranks are authorized by Congress, each branch of the uniformed services selects, manages, and utilizes warrant officers in slightly different ways. For appointment to warrant officer one (W-1), a warrant is approved by the secretary of the respective service. For chief warrant officer ranks (W-2 to W-5), warrant officers are commissioned by the President of the United States and take the same oath as regular commissioned officers (O-1 to O-10).

A warrant officer (WO) in the British Armed Forces is a member of the highest group of non-commissioned ranks, holding the Queen's (or King's) warrant, which is signed by the Secretary of State for Defence. Warrant officers are not saluted as they do not hold the Queen's Commission, however they are to be addressed as 'Sir/Ma'am' by subordinates. Commissioned officers may address warrant officers either by their appointment (e.g. QMSI, RSM or sergeant major) or as "Mister", "Mrs", or "Ms" and then their last name, e.g. "Mr Smith". Although often referred to along with non-commissioned officers (NCOs), they are not NCOs, but members of a separate group (traditional official terminology for the personnel of a unit is "the officers, warrant officers, non-commissioned officers and men"), although all have been promoted from NCO rank.

Royal Navy

Use of the term 'warrant officer' dates from the beginnings of the Royal Navy, a time when ships were commanded by noblemen who depended on others with specialist skills to oversee the practicalities of life on board. Specialists such as a ship's carpenter, boatswain and gunner were vital to the safety of all on board, and were accordingly ranked as officers - though by warrant rather than by commission. These and other specialists retained their distinctive rank and status until 1949, when the rank of warrant officer was abolished.