Phillip Chapman Lesh (born March 15, 1940) was a founding member of the band Grateful Dead, and played bass guitar in the band from 1965 to 1995.

Lesh started out as a trumpet player with a keen interest in avant-garde classical music and free jazz; he studied under the Italian modernist Luciano Berio (a classmate was the minimalist composer Steve Reich). While still a college student he met then-bluegrass banjo player Jerry Garcia. They formed a friendship and eventually Lesh was talked into becoming the bass guitarist for Garcia's new rock group, then known as the Warlocks. He joined them for their third or fourth gig (memories vary) and stayed until the end.

Lesh had never played bass before joining the band, which meant he learnt "on the job", but also meant he had no preconceived attitudes about the instrument's traditional "rhythm section" role. Indeed, he has said that his playing style was influenced more by Bach counterpoint than any rock or soul bass player (although one can certainly hear the fluidness and power of a Charles Mingus or Jimmy Garrison in Lesh's work). In a great Dead jam, Lesh's bass is as much a lead instrument as Garcia's guitar.

Lesh was never a prolific composer or singer with the Grateful Dead, although those songs he did contribute -- "Box of Rain", "Unbroken Chain", "New Potato Caboose", "Childhood's End", etc -- are among the best-loved in the band's repertoire. His interest in avant-garde musics was a crucial influence on the Dead, pushing them into new territory, and he was an essential part of the group and its mystique, best summed-up in the Dead Head truism: "If Phil's on, the band's on".

After the disbanding of the Grateful Dead Lesh continued to play with its offshoots The Other Ones and The Dead, as well as performing as Phil Lesh and Friends (one memorable tour paired him with Bob Dylan) and running the charitable Unbroken Chain Foundation.

Phil Lesh and wife Jill have two sons.