Of all the Burgundian aristocrats depicted by Rogier van der Weyden, one of the last whose identity remains a mystery is the man in the Brussels Museum who has come to be known as “Laurent Froimont”. To date, nobody has succeeded in securing the exact identity of this figure, who seems to have been close to the Burgundian court. The discovery of an unpublished book of hours, preserved in the Princeton University Library and illuminated in the entourage of Jacquemart Pilavaine, a miniaturist from Mons, invites us to re-examine this question. The margins of this manuscript contain the motto RAISON LENSEIGNE, which also appears in the painted portrait. Furthermore, the borders of the manuscript display coats-of-arms that allow for the identification of the original owners of the book: Jean de Froimont, counsellor of the feudal court of Hainaut, and Jeanne de le Croix, member of the Mons aristocracy. Froimont undertook several missions for the dukes of Burgundy, Philip the Good and Charles the Bold. He occupied an important place in Hainaut society during the third quarter of the 15th century, and might have been related, through marriage, to van der Weyden himself. This discovery also invites re-examination of the dating of the Brussels panel, as well as its traditional attribution to Rogier. It provides evidence of the strong ties that the famous painter continued to maintain with Hainaut, even once he was based in Brussels.