Tenacious D is an American comedy rock duo formed in Los Angeles in 1994 by the actors Jack Black and Kyle Gass. Their music showcases Black's theatrical vocal delivery and Gass' acoustic guitar playing. Critics have described their fusion of vulgar absurdist comedy with rock music as "mock rock". Their songs discuss the duo's purported musical and sexual prowess, their friendship and cannabis usage, in a style critics have compared with the storyteller-style lyrics of rock opera.

Prior to the release of the 2001 debut album Tenacious D, the duo had a three-episode TV series released on HBO between 1997 and 2000. The series came about after they met David Cross on the LA music scene and Black featured in episodes of Mr. Show with Bob and David. The band befriended the musician Dave Grohl, which began a relationship between Grohl's band the Foo Fighters and Tenacious D. Towards the end of the 1990s, the duo supported large rock acts such as Weezer, Pearl Jam, Tool, and Beck.

In 2000, they signed with Epic Records and the year after they released Tenacious D, their debut album featuring a full band, including Grohl. Their second single, "Tribute", achieved cult status, contributing to their popularity in the UK, Sweden and Australia. In 2003, the band released The Complete Master Works, a concert DVD, which was certified platinum. In 2006, the duo starred in a film, Tenacious D in The Pick of Destiny, and released the soundtrack as their second album, The Pick of Destiny. The film was a box office failure and received mixed reviews, but the Pick of Destiny tour was successful, with performances at venues including Madison Square Garden.

Following years of sporadic festival dates and special appearances, the band released their third album, Rize of the Fenix, in 2012, making light of The Pick of Destiny's commercial failure. In 2013, Tenacious D hosted the comedy music festival Festival Supreme in LA. In 2014, they featured on Ronnie James Dio – This Is Your Life, a compilation album to raise funds for cancer. Their cover of "The Last in Line" won the Grammy Award for best heavy metal performance at the 57th Annual Grammy Awards. In 2018, the band began independently releasing an animated hand-drawn web series, Tenacious D in Post-Apocalypto, and released the album Post-Apocalypto. In July 2024, Tenacious D cancelled the remainder of their world tour and put their work on hold after Gass made a controversial on-stage joke about the attempted assassination of Donald Trump in Pennsylvania.