Boulogne was also a major participant in the First Crusade; Eustace III of Boulogne's brothers, Godfrey of Bouillon and Baldwin of Bouillon, both became kings of Jerusalem, and Eustace himself was offered but declined the title.
Boulogne juga merupakan partisipan utama di dalam Perang Salib Pertama; saudara-saudara Eustace III dari Boulogne, Godfrey dari Bouillon dan Baldwin dari Bouillon, keduanya menjadi raja Yerusalem, dan Eustace sendiri ditawarkan namun ia menolak gelar tersebut.