Folie à Deux ([fɔli a dø]; French for "A Madness Shared by Two") is the fourth studio album by American rock band Fall Out Boy, released on December 10, 2008, by Island Records. As with their previous two albums From Under the Cork Tree (2005) and Infinity on High (2007), its music was composed by lead vocalist and guitarist Patrick Stump, with lyrics penned by bassist Pete Wentz. Regarding the writing process, the band considered Folie à Deux to be their most collaborative record.

Unlike their prior releases, the album was recorded in relative secrecy with producer Neal Avron from July to September 2008. The recording sessions inspired lyricism relating to decaying relationships, moral dilemmas, and societal shortcomings, many with a political edge. The album's style moved away from early emo power chords and toward a wider variation in genres. Fall Out Boy recruited several guest artists for Folie à Deux, as well as employing instruments and recording techniques previously unfamiliar to the group. To promote the album, the band launched a viral campaign based around a Big Brother-type organization named "Citizens For Our Betterment" and embarked on an extensive tour schedule.

Folie à Deux debuted at number eight on the US Billboard 200, selling over 149,000 copies in its first week of sales, although it was less commercially successful than Infinity on High. The album received favorable reviews from critics, with many focused on the creativity and various styles touched on while others expressed concern that it was overly indulgent. As of 2013, Folie à Deux has sold over 449,000 copies in the United States.