This article uses a legal case from the French Indian Ocean empire during the American Revolutionary War (ca. 1778–1780) to explore the operation of mobile legal spaces: that is, law-filled spaces that moved (such as ships) or which were highly shaped by movement in and out of them (ports). These types of spaces, characteristic of early modern seaborne European empires, created distinctive problems and opportunities for those navigating imperial rule, both rulers and ruled. In particular, the essay argues that the mobility of legal spaces added significant spatial and temporal dimensions to the well-known legal diversity and pluralism of the early modern world. In mobile spaces, legal regimes might overlap or conflict temporarily, be relevant for only a fixed period of time, or come into force when a ship reached a particular location. Recognising the ubiquity of these mobile legal spaces and the powerful effects that they had can change how we understand the functioning of law in colonial empires.