This article considers the construction of indigenous (indio) slave identity within the contexts of the sixteenth-century Atlantic and Mediterranean worlds. Of the more than two thousand indio slaves from Latin America who were forced to migrate to Castile during the sixteenth century, nearly two hundred questioned the legality of their enslaved condition based upon the tenets of the New Laws (1542), which declared indios to be free vassals of the Spanish Crown. This resulted in the initiation of 123 before the tribunals of the House of Trade and the Council of the Indies. Because of the need to identify the imperial (Spanish versus Portuguese) origins of indio litigants, witnesses navigated physiognomic identity markers and other criteria. But identification was a subjective art that depended on the legal culture of the courtroom, the experiences and incentives of deponents, and the presence of other slaves in Castile from Brazil, West and North Africa, South and East Asia, or Granada. Analyzing the complex process of labeling indios in such a globalized context shows not only how notions of indigenous enslaveability evolved over the course of the sixteenth century but how Castilian interpretations of phenotype and identity were varied and complex.