A warrant officer (WO) in the British Armed Forces is a member of the highest-ranking group of non-commissioned ranks, holding the King's Warrant, which is signed by the Secretary of State for Defence.

Warrant officers are not saluted, because they do not hold the King's Commission, but are addressed as "Sir" or "Ma'am" by subordinates. Commissioned officers may address warrant officers either by their appointment (e.g. sergeant major) or as "Mister", "Mrs" or "Ms" along with their last name. 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.

In November 2018, the most senior warrant officer and most senior other ranks position was created, titled Senior Enlisted Advisor to the Chiefs of Staff Committee. A warrant officer in this position is the most senior warrant officer in the British Armed Forces.

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.

In 1971, warrant officers reappeared in the Royal Navy, but these appointments followed the Army model, with the new warrant officers being classified as ratings rather than officers, superior to the rate of chief petty officer. They were ranked as equivalents to warrant officer class I in the British Army and Royal Marines and with warrant officers in the Royal Air Force. The rate was initially titled as fleet chief petty officer, becoming warrant officer in 1985.

In April 2004, the Royal Navy created the rate of warrant officer 2 (WO2), superior to the CPO and subordinate to existing warrant officers who were retitled as warrant officer 1 (WO1). The WO2 replaced the non-substantive appointment of charge chief petty officer (CCPO) in the technical branches. Prior to this change, a CCPO was classified as a NATO OR-8, equivalent to WO2. Royal Navy warrant officers wear the same insignia as their counterparts in the Royal Marines.

In 2005, the Royal Navy introduced the appointment of executive warrant officer (EWO) in all ships and shore establishments. The EWO is the senior warrant officer within the unit, and a member of the senior command team. The appointment is intended to be filled by an experienced WO1. Above these were five command warrant officers: CWO Surface Ships, CWO Submarines, CWO Royal Marines (subordinate to the Corps RSM), CWO Fleet Air Arm and CWO Maritime Reserves. Under the Navy Command Transformation Programme, there are now a Fleet Commander's Warrant Officer and a Second Sea Lord's Warrant Officer, all working with the Warrant Officer of the Naval Service, taking over the roles of the command warrant officers.

The most senior warrant officer is the Warrant Officer to the Royal Navy (WORN). This post replaced the command warrant officer working under the Second Sea Lord in 2010.