William Cox (born October 18, 1941) is an American bassist, best known for performing with Jimi Hendrix. Cox is the only living musician to have regularly played with Hendrix: first when both were in the Army, then in 1969 with the interim group that backed Hendrix at Woodstock (informally referred to as "Gypsy Sun and Rainbows"), followed by the trio with drummer Buddy Miles that recorded the live Band of Gypsys album, and, lastly, The Cry of Love Tour trio with Mitch Mitchell back on drums. Cox continues to perform dates with the Band of Gypsys Experience and the Experience Hendrix Tour.

In addition to Hendrix, he has either been a member of the house or touring band or recorded sessions for Sam Cooke, Slim Harpo, Joe Simon, Charlie Daniels, John McLaughlin, Rufus Thomas, Carla Thomas, Lou Rawls, Etta James, Jackie Wilson and Little Richard.

Born in Wheeling, West Virginia, Billy Cox was raised in Pittsburgh, Pennsylvania, and attended Schenley High School.

Cox met Jimi Hendrix when they were serving in the Army at Fort Campbell, Kentucky, in 1961. While using the bathroom at Service Club No. 1 during a sudden rainstorm, he heard guitar playing inside. Impressed with what he heard, he introduced himself, told Hendrix he played bass, and they were jamming soon after. They became, and remained, fast friends. They left the military around the same time and they played clubs around Clarksville, Tennessee, finally moving to Nashville. They formed a group called the King Kasuals, and played at several Nashville clubs, mostly the Del Morocco. Occasionally the band would play outlying gigs in the southeast, once as far north as Indianapolis playing what was called the "Chitlin' Circuit". Cox and Hendrix also played in the backing band for Marion James around this time.

Hendrix eventually left Nashville, playing all over the U.S. in the backing groups of several famous artists (most notably Little Richard and the Isley Brothers) until he was "discovered" by Chas Chandler in New York City's Greenwich Village. Chandler took Hendrix to England, but before Hendrix left, he called Cox and asked him to join him. As Cox "only had three strings on [his] bass" and no money to travel to New York, he simply thanked Hendrix and wished him well.

During this period, Cox played bass on such pioneering R&B television shows as Nashville's "Night Train" and "The!!!! Beat" from Dallas, Texas, working closely with Hoss Allen and John Richbourg of WLAC Radio.

In 1969, several months before bassist Noel Redding left the Jimi Hendrix Experience, Hendrix called his old friend Cox, who joined him in New York as his studio bassist. Following the break-up of the Experience, Cox performed with Hendrix's interim group, informally called Gypsy Sun and Rainbows. This group played at Woodstock and two low-key New York gigs. Hendrix then formed another short-lived group with Cox and Buddy Miles, that recorded Band of Gypsys, the live album that he owed former manager Ed Chalpin as part of a legal settlement. Following their demise, Cox recorded and toured with Hendrix and Mitch Mitchell for The Cry of Love Tour, when promoters sometimes billed them as the "Jimi Hendrix Experience".

Cox contributed most of the bass parts on the first group of posthumously released Hendrix albums, including The Cry of Love (1971), Rainbow Bridge (1971), War Heroes (1972), and Loose Ends (1974). Most of the tracks from these albums were consolidated as First Rays of the New Rising Sun (1997), which is the most complete attempt to present Hendrix's fourth studio album (see Hendrix's posthumous discography and videography for a complete list). Additionally, Cox appears on many live albums and films from the Cry of Love Tour.