Abstract Prize law was a legal regime that played a crucial role in maritime trade and warfare in the European imperial world before the twentieth century, governing both the capture and disposition of enemy property seized by belligerents at sea during wartime. Prize law outlined the rules by which captures were to take place and how captured property was to be handled, adjudicated, and (if “condemned” or deemed a valid capture), disposed of. All European maritime powers had prize regimes during the early modern era, which collectively adjudicated the fates of tens of thousands of vessels worth the equivalent of hundreds of millions of today’s dollars. This article examines the procedural law of prize in the long eighteenth century: the rules that governed how legal actors in the prize regime brought cases before tribunals and the tribunals’ rules for adjudicating them. It shows that the law of prize was an interpolity legal regime, generated within the ambiguous legal spaces that existed between the jurisdictional boundaries of individual states and empires. The article focuses on three important areas of procedural law for evidence of the interpolity character of the prize regime. It looks first at the distinctive role played by mariners in prize procedures. Captains of privateers and other capturing vessels functioned as quasi-officers of the court, responsible for assembling the documentary record that prize tribunals used to adjudicate captures and bringing the cases before the courts. Imperial prize regulations gave almost no guidance on the rules that they were to follow. The proper procedures were defined by mariners and merchants themselves, as unwritten rules articulated at sea. Second, we turn to how the prize tribunals managed language diversity and translation. I show that, a few exceptions aside, prize tribunals did not acknowledge linguistic difference among litigants. Instead, the tribunals relied on silent and often unacknowledged translation processes, which embedded in their proceedings a fiction that all of the actors before them communicated in a common language. Last, the article shows that prize tribunals applied foreign law in their proceedings, in ways that were not formally authorized by imperial law. By focusing on the neglected procedural law of prize, this article offers a different view of the prize regime than the one that has been painted by many studies of substantive law of prize in the long eighteenth century, in at least two ways. One is that studying the procedural law of prize casts into sharp relief the bottom-up character of prize law. Far from being dictated by sovereigns or from imperial centers, much about the prize regime was created informally by lower-level legal actors. Second, a focus on procedural law reveals important continuities in prize law across space and time. Substantive prize law during the period saw extensive debates about the nature of neutral rights, among other issues, with different empires articulating radically different doctrines. The procedural law of prize, true to its interpolity origins, remained remarkably constant across empires and over time.