John Edward Illsley (born 24 June 1949 in Leicester) is an English musician who rose to fame as the bass guitarist of the critically acclaimed rock band Dire Straits. With Dire Straits, Illsley has been the recipient of multiple BRIT and Grammy Awards and a Heritage Award.

As one of the founding band members, with brothers and guitarists David and Mark Knopfler, and drummer Pick Withers, Illsley played a role in the development of Dire Straits' sound. When David Knopfler left the band in the early 1980s, Illsley not only lost a bandmate, but also a former roommate and good friend. By the time the group disbanded in 1995, with changes in personnel, Illsley and frontman Mark Knopfler were the only two original band members remaining.