John Martin McDonnell is a British politician who served as Shadow Chancellor of the Exchequer from 2015 to 2020. A member of the Labour Party, he has been Member of Parliament (MP) for Hayes and Harlington since 1997.

The Shadow Chancellor of the Exchequer in the British Parliamentary system is the member of the Shadow Cabinet who is responsible for shadowing the Chancellor of the Exchequer. The title is given at the gift of the Leader of the Opposition and has no formal constitutional role, but is generally considered the second-most senior position on the opposition frontbench, after the Leader. Past Shadow Chancellors include Harold Wilson, James Callaghan, Edward Heath, Geoffrey Howe, Kenneth Clarke, Gordon Brown, and John McDonnell.