One Canada Square is a skyscraper in Canary Wharf, London. It is the third tallest building in the United Kingdom at 770 feet (235 m) above ground level, and contains 50 floors. It achieved the title of the tallest building in the UK upon completion in 1991 and held the title for 21 years until the completion of The Shard (310m) in 2012.

One Canada Square was designed by César Pelli with Adamson Associates and Frederick Gibberd Coombes. The building is clad with stainless steel. One of the predominant features of the building is the pyramid roof, which contains a flashing aircraft warning light, a rare feature for buildings in the United Kingdom. The distinctive pyramid pinnacle is 800 feet (240 m) above sea level.

One Canada Square is primarily used for offices, though there are some retail units on the lower ground floor. There is no observation floor. It is a prestigious location for offices and as of October 2017 was completely let. The building is recognised as a London landmark, and it has gained much attention through film, television, and other media as one of the tallest buildings in the United Kingdom.

The original plans for a business district on Canary Wharf came from G Ware Travelstead. He proposed three 260 m (850 ft) towers. Travelstead was unable to fund the project, so the plans were sold to Olympia & York in 1987. Olympia & York grouped all three towers into an area known as Docklands Square, and the main tower was designated DS7 during planning. Docklands Square was later renamed Winston Square before finally being renamed as Canada Square.

The architects chosen to design One Canada Square were César Pelli & Associates, Adamson Associates, and Frederick Gibberd Coombes & Partners. They designed the tower with a similar shape to 200 Vesey Street (formerly Three World Financial Center), New York City, which was also developed by Olympia & York and designed by Argentine architect César Pelli. The shape was also made reminiscent of "Big Ben". Olympia & York wanted to clad One Canada Square in stone, just like Peli's World Financial Center towers constructed in 1986, but the architects first wanted to use aluminium for its low density, before insisting on steel to reflect Britain's heritage as an industrial nation. The final steel clad chosen was Patten Hyclad Cambric-finish stainless-steel.

Commenting on the reason for choosing steel for the clad:

"We studied the cladding material carefully and chose stainless steel with a linen finish because it seemed to fit the atmosphere of London."

One Canada Square was originally designed to be 864 feet (263 m) high at 55 storeys, but that penetrated the permitted projection height of the flight obstruction area of the airport approach district to London City Airport, but this was extended to a height of 30 feet (9.1 m) above kerb level in consideration of the fact that One Canada Square was on the external zone of the airport approach. To comply with air traffic safety regulations, the architects took five floors off the tower. The final height of 824 feet (251 m) was permitted, otherwise, the developers would have had to dismantle what was necessary to fit the height restriction. After losing five floors, Olympia & York insisted the other floors had to make up the lost floor space by increasing mass to the remaining floor space which created a tower that was not as slim as Pelli desired. Pelli and the other architects proposed alternatives, such as building more floors below ground and creating an extension of the tower into Docklands Square, which were similar ideas based upon previous Olympia & York buildings, though the ideas were rejected as it did not fit the basis of prime office space.