The Century 21 Exposition (also known as the Seattle World's Fair) was a world's fair held April 21, 1962, to October 21, 1962, in the American city of Seattle, Washington.

Nearly 10 million people attended the fair.As planned, the exposition left behind a fairground and numerous public buildings and public works; some credit it with revitalizing Seattle's economic and cultural life (see History of Seattle since 1940). The fair saw the construction of the Space Needle and Alweg monorail, as well as several sports venues (Washington State Coliseum, now Climate Pledge Arena) and performing arts buildings (the Playhouse, now the Cornish Playhouse), most of which have since been replaced or heavily remodeled. Unlike some other world's fairs of its era, Century 21 made a profit.

The site, slightly expanded since the fair, is now called Seattle Center; the United States Science Pavilion is now the Pacific Science Center. Another notable Seattle Center building, the Museum of Pop Culture (earlier called EMP Museum), was built nearly 40 years later and designed to fit in with the fairground atmosphere.