John Francis Anthony Pastorius III, also known as Jaco Pastorius (; December 1, 1951 – September 21, 1987) was an American jazz bassist, composer, and producer. Widely regarded as one of the greatest and most influential bassists of all time, Pastorius recorded albums as a solo artist, band leader, and as a member of the jazz fusion group Weather Report from 1976 to 1981. He also collaborated with numerous artists, including Herbie Hancock, Pat Metheny and Joni Mitchell.

His bass style was influenced by funk and employed the use of fretless bass, lyrical solos, bass chords and innovative use of harmonics. As of 2017, he was the only one of seven bassists inducted into the DownBeat Jazz Hall of Fame to have been known for their work on the electric bass, and he has been lauded as among the best bassists of all time.

Pastorius suffered from drug addiction and mental health issues and, despite his widespread acclaim, over the latter part of his life he had problems holding down jobs due to his unreliability. In frequent financial difficulties, he was often homeless in the mid-1980s. He died in 1987 as a result of injuries sustained in a beating outside a South Florida after-hours nightclub.

Since his death in 1987, his work has continued to be widely influential. He was elected to the DownBeat Hall of Fame in 1988 and was the subject of the 2014 documentary film Jaco.