Demon Days is the second studio album by the British virtual band Gorillaz. It was released on 11 May 2005 in Japan and 23 May 2005 in the United Kingdom by Parlophone and in the United States by Virgin Records. Produced by Gorillaz, Danger Mouse, Jason Cox, and James Dring, it features De La Soul, Neneh Cherry, Martina Topley-Bird, Roots Manuva, MF Doom, Ike Turner, Bootie Brown of the Pharcyde, Shaun Ryder and Dennis Hopper.

As with the band's eponymous 2001 debut, Demon Days and its performances were accompanied by various multimedia, including interactive features on the Gorillaz website, animated music videos and animatics. The visuals were designed by the Gorillaz co-creator Jamie Hewlett, under his design company Zombie Flesh Eaters.

Demon Days reached the top 10 in 23 countries. It reached number one on the UK Albums Chart and number 6 on the US Billboard 200, and was later certified six times platinum in the UK and double platinum in the US. Outperforming their debut, the album has sold eight million copies worldwide. The album spawned the singles "Feel Good Inc.", "Dare", "Dirty Harry", "Kids with Guns", and "El Mañana".

Spin ranked Demon Days the fourth-best album of 2005, while Mojo ranked it at number eighteen on their year-end list and hailed the album as a "genre-busting, contemporary pop milestone". The NME placed the album at number 98 on their list of 100 greatest albums of the decade, and Uncut placed it at number 75 on their list of top 150 albums of the decade. Complex included it on their list of 100 Best Albums of the Complex Decade, placing it at number 43, and Spin later included it in their list of "The 300 Best Albums of 1985–2014". In 2020, the album was included at the 100 Best Albums of the 21st Century list of Stacker, being ranked at 25. In 2023, Rolling Stone ranked Demon Days number 437 on their list of the 500 Greatest Albums of All Time.