The 63 Building, officially called 63 SQUARE, is a skyscraper on the island of Yeouido, overlooking the Han River in Seoul, South Korea. At 249.6 m (819 ft) high, it was the tallest building outside North America when it was opened on July 27, 1985, and it is the second-tallest gold-clad building in the world after Grand Lisboa in Macau. It stood as South Korea's tallest building until the Hyperion Tower surpassed it in 2003, but remained the country's tallest commercial building until the Northeast Asia Trade Tower was topped-out in 2009.

Yeouido is a large island on the Han River in Seoul, South Korea. It is Seoul's main finance and investment banking district. It is 8.4 square kilometres (3.2 sq mi) large and has a population of 32,674 as of August 2023.