This essay draws political theory into dialogue with recent work in economic history and the history of capitalism to develop an account of the unique injustice produced by capitalist slavery in the antebellum United States. Prevailing approaches to thinking about slavery in political theory tend to disembed it from its broader socioeconomic context, which has led theorists to overlook some of the distinctive horrors associated with capitalist slavery in particular. In response, I develop a theory of capitalist slavery as expropriation, conceived as violent domination harnessed to the imperatives of capital accumulation. Capitalist slavery-as-expropriation encompasses two analytically distinct moments: the moment of confiscation, in which human lives and capacities are enclosed via commodification, and the moment of conscription, in which enslaved labor is mobilized via routine violence. Though enslaved people were not market subjects, this framework reveals the extent to which they were nevertheless subject to the market.