David Ellefson is an American musician, best known for his long tenure as the bassist and backing vocalist for heavy metal band Megadeth across two stints.

Ellefson initially became an accomplished bassist and honed his songwriting skills while leading several of his own bands through the club scene of North America's Midwest region before relocating to Los Angeles. He was then the bassist of Megadeth from 1983 to their breakup in 2002, and again from 2010 to 2021.

Ellefson co-founded the hard rock band the Lucid in 2021 alongside vocalist Vinnie Dombroski (Sponge), guitarist Drew Fortier (formerly of Bang Tango), and drummer Mike Heller (Fear Factory, Malignancy, Raven). In addition to playing bass guitar in Megadeth and the Lucid, Ellefson had various side projects, which include Temple of Brutality, F5, Killing Machine, and Metal Allegiance, and he is also the current bassist of Metal Church.

Ellefson is of Norwegian ancestry.

Ellefson played in a band called Killers, which featured himself on bass, Greg Handevidt on guitar, Brett Frederickson on drums and Jerry Giefer, during Frederickson's senior year of high school.

Other than guitarist, vocalist, and primary lyricist Dave Mustaine, Ellefson was the only constant member of Megadeth from early in the band's history in 1983 until the group's dissolution in 2002. Ellefson has maintained that he was a founding member of the band, though Mustaine has disputed the idea in multiple interviews. Mustaine referred to Ellefson as "Dave Junior", which was often shortened to "Junior" after vocalist Lor Kane used the nickname to differentiate between the two Daves. Ellefson appeared on every album and tour from 1985's record Killing Is My Business... And Business Is Good! up until 2002's Rude Awakening. Mustaine reformed Megadeth without Ellefson in 2004. In the early days of Megadeth, Ellefson played using his fingers, but as Megadeth progressed and the music became more complex, he tended to prefer playing with a pick, or plectrum. Ellefson was credited with some Megadeth songs, including "Family Tree" from 1994's Youthanasia (in the 2004 remastered collection of the CD, however, the track is credited to Mustaine, Ellefson, Friedman, Menza, but Mustaine credited the Rust in Peace-era line-up with the entire Youthanasia album as a tribute to the band's success at the time of the record's initial release).

Ellefson formed F5 following Megadeth's 2002 disbandment. The band featured Ellefson on bass, Dale Steele on vocals, Steve Conley on lead guitar, John Davis on rhythm guitar, and former Megadeth bandmate Jimmy DeGrasso on drums. Their first album, A Drug for all Seasons, was released in 2005. F5 appeared in support for Disturbed in February 2006 and toured the American Mid-West in the summer of 2006. Ellefson also appeared on the new Killing Machine record Metalmorphosis in 2006 alongside DeGrasso; he has also been working with Temple of Brutality. Ellefson is currently a member of the melodic power metal band Avian, which features singer Lance King. He commented in an interview with Alternative-Zine.com that "Megadeth was really just a starting point for me, creatively". Ellefson played five tracks for the Soulfly album Prophecy and also played on one track on Dark Ages. He also worked with underground emcee/Record producer Necro for his album entitled Death Rap.

Ellefson is also a member of a cover band called Hail!. Ellefson, DeGrasso (Mike Portnoy for 2010), Tim "Ripper" Owens and Andreas Kisser formed the band in late 2008. The band was, as of April 2009, touring Europe. Ellefson participated in the Christmas Rock project Northern Light Orchestra, he performed bass on many of the 17 tracks produced by rock drummer Ken Mary and also at the live show Christmas in April Orpheum Theatre, Phoenix, Arizona.