Eustace the Monk (Old French: Eustache le Moine; c. 1170 – 24 August 1217), born Eustace Busket, is a mercenary and pirate, in the tradition of medieval outlaws.

The birthplace of Eustace was not far from Boulogne.

A 1243 document mentions a Guillaume le Moine, seigneur de Course, which indicates that the family lived in that vicinity.