This article examines colonial legal categories such as “national status” and “coastwise shipping” that shaped the movement of goods and people between U.S. colonies and the metropole. Focused on the case of Puerto Rican migration to the U.S. mainland in the early twentieth century, it argues that these legal categories conditioned migration patterns and that migrants, in turn, actively shaped new legal categories. Drawing on sources from both U.S. and Puerto Rican archives, this article contributes to an emerging body of literature on U.S. imperialism, law, and migration in the Americas. It shows that colonial legal categories are critical to understanding enduring migration streams to the United States that have long been embedded in imperial relationships.