This article focuses on an influential but today little-known book first published in Bordeaux in 1647 and later reprinted in multiple editions: Étienne Cleirac’s Us et coustumes de la mer (Usages and Customs of the Sea). It is the earliest and most extensive vernacular treatise on maritime law composed in early modern Europe, assembling and commenting on a large body of legal norms issued from the twelfth century to the then present, including the Laws of Wisby, the Judgements of Oléron, and a great many French regional and royal compilations of maritime law. The importance of this text is threefold. At the most basic, it helps us reconstruct the process through which doctrine and scholarship on European maritime law developed in the seventeenth and eighteenth centuries. It also brings new evidence to bear on controversial scholarly debates about the so-called lex mercatoria, its supposedly private and cosmopolitan nature, and its transition from the medieval to the early modern period. Finally, it sheds new light on the intersection of law, politics, and socio-economic transformations in France during the half-century before the proclamation of the Ordonnance de commerce (1673) and the Ordonnance de la marine (1681).