Eustace III, was a count of Boulogne, successor to his father Count Eustace II of Boulogne. His mother was Ida of Lorraine. His father Eustace II appeared at the Battle of Hastings in 1066 as an ally of William the Conqueror, and is listed as a possible killer of Harold II; he is also believed to have given William his own horse after the duke's was killed under him by Gyrth, brother of Harold. Eustace III succeeded as Count of Boulogne in 1087.