The Royal Westminster Militia, later the 5th (Reserve) Battalion, Royal Fusiliers, was an auxiliary regiment raised in the City of Westminster in the suburbs of London. Descended from the Westminster Trained Bands, which were on duty during the Armada campaign of 1588 and saw considerable service during the English Civil War, the regiment underwent reorganisation in 1760 as part of the Middlesex Militia. From 1778 until 1918 the regiment served in home and colonial defence in all of Britain's major wars, and supplied thousands of reinforcements to the Royal Fusiliers during World War I. After a shadowy postwar existence the unit was finally disbanded in 1953.

The Westminster Trained Bands were a part-time military force established in 1572, recruited from residents of the City of Westminster. As part of the larger London Trained Bands, they were periodically embodied for home defence, such as during the 1588 Spanish Armada campaign. Although service was technically restricted to London, the Trained Bands formed a major portion of the Parliamentarian army in the early years of the First English Civil War. After the New Model Army was established in April 1645, they returned to their primary function of providing security for the palaces of Westminster and Whitehall. Following the 1660 Stuart Restoration, the City of London Militia Act 1662 brought them under the direct control of the Crown, with the Trained Bands becoming part of the British Army.