John Fenwick, real surname Caldwell (1628–20 June 1679) was an English Jesuit, executed at the time of the fabricated Popish Plot.

His correspondence was seized, but to the Crown's disappointment it turned out to be completely innocuous: as he forcefully pointed out at his second trial, among at least a thousand letters taken from him there was not one which could be construed as treasonable.

He was tried for high treason with William Ireland, in that they had conspired to kill King Charles II, a charge fabricated by Oates and later embellished by other informers.

As an act of clemency, the King, who was well aware that they were innocent, but realised that it would be politically unthinkable to grant a royal pardon or reprieve, ordered that they be allowed to hang until they were dead, and be spared the indignity of drawing and quartering.

[7] In a sign that public sympathy was turning against the plot, the large crowd heard their final speeches from the scaffold in respectful silence, as each maintained his innocence.

[2] An account of the trial and condemnation of the five Jesuits for High Treason, in conspiring the Death of the King, the Subversion of the Government and Protestant Religion was published by authority at London, 1679.