The SR Merchant Navy class (originally known as the 21C1 class, and later informally known as Bulleid Pacifics, Spam Cans – which name was also applied to the Light Pacifics – or Packets) is a class of air-smoothed 4-6-2 (Pacific) steam locomotives designed for the Southern Railway by Oliver Bulleid. The Pacific design was chosen in preference to several others proposed by Bulleid. The first members of the class were constructed during the Second World War, and the last of the 30 locomotives in 1949.

Incorporating a number of new developments in British steam locomotive technology, the design of the Merchant Navy class was among the first to use welding in the construction process; this enabled easier fabrication of components during the austerity of the war and post-war economies. In addition, the locomotives featured thermic syphons in their boilers and the controversial Bulleid chain-driven valve gear. The class members were named after the Merchant Navy shipping lines involved in the Battle of the Atlantic, and latterly those which used Southampton Docks: a publicity masterstroke by the Southern Railway, which operated Southampton Docks during the period.

Due to problems with some of the more novel features of Bulleid's design, all members of the class were modified by British Railways during the late 1950s, losing their air-smoothed casings in the process. The Merchant Navy class operated until the end of Southern steam in July, 1967. A third of the class has survived and can be seen on heritage railways throughout Great Britain. They were known for reaching speeds of up to 105 mph (167 km/h); such speeds were recorded by examples including No. 35003 Royal Mail (since scrapped) and Nos. 35005 Canadian Pacific and 35028 Clan Line (both preserved).