Who Wants to Be a Millionaire (colloquially referred to as simply Millionaire) is an American television game show based on the format of the same-titled British program created by David Briggs, Steven Knight and Mike Whitehill and developed in the United States by Michael Davies. The show features a quiz competition with contestants attempting to win a top prize of $1,000,000 by answering a series of multiple-choice questions, usually of increasing difficulty. The program has endured as one of the longest-running and most successful international variants in the Who Wants to Be a Millionaire? franchise.

The original American version premiered on ABC on August 16, 1999, as part of a two-week daily special event hosted by Regis Philbin. After this and a second two-week event aired in November 1999, ABC commissioned a regular series that launched on January 9, 2000, and ran until June 27, 2002. Philbin hosted the entire run of the original network series as well as two additional special event series that aired on ABC in 2004 and 2009.

A daily version of Millionaire produced for syndication began airing on September 16, 2002, and was initially hosted by Meredith Vieira. Cedric the Entertainer took over the show in 2013 following Vieira's departure, with Terry Crews replacing him in 2014. The final host of the syndicated series was Chris Harrison, who took over from Crews in 2015 and hosted until the show was canceled, with the finale airing on May 31, 2019. On January 8, 2020, seven months after the cancellation was announced, ABC renewed the show for a 20th Anniversary season, hosted by Jimmy Kimmel (who is also a co-executive producer of the show) with celebrity contestants. The anniversary season premiered on April 8, 2020, and its success led to the show being renewed for a second season with Kimmel, which premiered on October 18, 2020. On May 3, 2024, ABC renewed the show for a 25th Anniversary season after a three-year hiatus, which premiered on July 10, 2024.

The show has had numerous format and gameplay changes over its runtime and, since its debut, twelve contestants and one team of two contestants (fourteen combined) have answered all questions correctly and won the top prize. As the first US network game show to offer a million-dollar top prize, the show made television history by becoming one of the highest-rated game shows in the history of US television. The US Millionaire won seven Daytime Emmy Awards, and TV Guide ranked it No. 6 in its 2013 list of the 60 greatest game shows of all time.