John Alec Entwistle (9 October 1944 – 27 June 2002) was an English singer, songwriter, musician, composer and record producer, best known as the bass guitarist for the rock band the Who. Entwistle's music career spanned over four decades. Nicknamed "The Ox" and "Thunderfingers", he was the band's only member with formal musical training and also provided backing and occasional lead vocals. Entwistle was inducted into the Rock and Roll Hall of Fame as a member of the Who in 1990.

Entwistle was the first member of the Who to begin a solo career in 1971 while still a member of the band, and has released seven solo studio albums, four compilation albums and two live albums. His best-selling studio album was Too Late the Hero (1981), and its title-track his best-selling single. Musicians who have played on his albums include the Who's Keith Moon, Peter Frampton of Humble Pie, Joe Walsh of the Eagles, Leslie West of Mountain, Vivian Stanshall, Neil Innes, Zak Starkey, Howie Casey, Dick Parry, Jimmy McCulloch of Paul McCartney and Wings, Joe Vitale and Tony Ashton.

Renowned for his musical abilities, Entwistle is widely regarded as one of the greatest and most influential rock bassists of all time. His instrumental approach featured pentatonic lead lines and a then-unusual treble-rich sound ("full treble, full volume"). He was voted as the greatest bass guitar player ever in a 2011 Rolling Stone readers' poll and, in 2020, the same magazine ranked him number three in its list of the "50 Greatest Bassists of All Time".