The Gunpowder Conspiracy of 1605, or the Powder Treason or Gunpowder Plot, as it was then known, was a failed assassination attempt by a group of provincial English Catholics against King James I of England and VI of Scotland.

The plot intended to kill the king, his family and most of the Protestant aristocracy quite literally in a single blow, by blowing up the Houses of Parliament during the State Opening on 5 November 1605.