Bonkyll Castle (also variously spelled Bonkyl, Boncle, Buncle, Bunkle or Bonkill) was a medieval fortress situated in the historic Scottish county of Berwickshire, from 1973 the Scottish Borders. It is situated 4 miles north of Duns and 4 miles south of Grantshouse. Few traces survive and the site is protected as a scheduled monument. It was the seat of a junior branch of the Stewart family, known as "Stewart of Bonkyl", from which was descended in another junior branch "Stewart of Darnley", the paternal family of King James I & VI of Scotland and England.