John Gibb Marshall (born 11 January 1953), better known by the stage name John Sessions, is a British actor and comedian. He is known for comedy improvisation in television shows such as Whose Line Is It Anyway?, as a panelist on QI, and as a character actor in numerous films, both in the UK and in Hollywood.
Sessions and Stephen Fry were the only two regular performers on the original radio broadcast of Whose Line Is It Anyway? in the late 1980s. When the show, still hosted by Clive Anderson, made the transition to television, Fry departed from regular appearances, but Sessions remained the featured panelist for the first season, a frequent player in the second, but he did not appear again after his two appearances in the third series.