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 Seattle, Washington, United States. Nearly 10 million people attended the fair during its six-month run.

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 (1940-present)).