A Regiment of the British army. Raised in 1678 in Scotland for service against the covenanting forces, as were the Royal Scots Greys, the Regiment gave continuous service until 1959 when it was amalgamated in 1959 with the Highland Light Infantry (City of Glasgow Regiment) to form The Royal Highland Fusiliers (Princess Margaret's Own Glasgow and Ayshire Regiment.

The Royal Scots Fusiliers were successively known as: The Earl of Mar's Regiment (or 'Mar's Grey Breeks'), the 21st Regiment of Foot, the Royal North British Fusiliers and the 21st Royal Scots Fuiliers. The most notable battle honours of the Regiment are: Blenheim, Dettingen, Inkerman, Gheluvelt (1914) and numerous honours from the Boer and First and Second world wars.

Famous officers of the Regiment include: Sir Andrew Agnew (in command at Dettingen) Field Marshall Sir Paul Haynes (in command at Inkerman), Field Marshall Lord Trenchard (founder of the Royal Air Force, Deneys Reitz one time deputy Prime Minister of South Africa and South African High Commissioner in London, Sir Winston Churchill who commanded the 6th Battalion of the Regiment (Territorial Army) on the Western Front for a few months in 1915 while in disgrace after Gallipoli (his second in command was Sir Archibald Sinclair later Air Minister)and Major General Hakewell Smith, commander of 52(Lowland) Division in WWII who had been one of Churchill's subalterns.

Two histories of the Regiment have been written:

The History of the Royal Scots Fusiliers 1678-1920 by John Buchan (Lord Tweedsmuir) and The Royal Scots Fusiliers 1920-1959 by Colonel J C Kemp.