Slave traders forced more than 1.65 million captive Africans aboard illegal transatlantic slave ships during the nineteenth century. This article focuses on the final phase of this brutal traffic, between 1850 and 1866. It argues that slave traders sustained their illicit industry, in large part, by strategically coordinating their financial arrangements against a rising tide of international suppression. One key tactic was for slave trade investors in the United States, Cuba, Africa, and Iberia to lower the risks of interdiction by joining forces and co-financing voyages. Another was to combine with an international cast of merchants and bankers, who helped them launder slave trade capital and transmit it to their distant allies. This capital was concealed within broader currents of global commerce, which was, in turn, spurred by the growth of free trade in the nineteenth century. These myriad alliances and capital flows undergirded the trade until its final extinction in the 1860s.