Ian AR Stewart (18 July, 1938–12 December, 1985) was a British rock musician.

Stewart played piano in the original line-up of The Rolling Stones. He predates both Bill Wyman and Charlie Watts as members of the band. Because the band's manager Andrew Loog Oldham didn't think Stewart's looks were good enough for publicity purposes, Stewart officially "left the group" but continued as their road manager and pianist.

Stewart, or "Stu" as he was known, played keyboards on most of the Stones' essential albums from the 1960s until the 1980s. He also accompanied the Stones on most of their tours during this time period. He is referred to as the Sixth Stone (although this title, as with its more famous counterpart, the Fifth Beatle, has some argument, as pianist Nicky Hopkins and saxophonist Bobby Keys also contend for the title).

The Led Zeppelin song "Boogie with Stu" (on their album Physical Graffiti) is a tribute to him and his famous boogie woogie piano-playing style; Stu is featured playing on the track.