This dissertation analyzes the deployment of ethnic labels in a diverse corpus of Thai-language compositions produced from the fifteenth century to the mid-nineteenth. Rather than seeing ethnic groups as objective entities, I uncover the ways in which Siam’s court officials and their associates increasingly invoked ethnonyms to label political categories to their advantage. In the seventeenth century, for instance, law texts indicate that officials had begun to mobilize ethnic labels to better manage an ever-more specialized labor organization. By the end of the eighteenth century, a variety of compositions reveal that writers had begun to apply ethnic labels to states and their affiliates, gradually supplanting older conventions of representing kingdoms as personal networks. Siam’s royal court then began to rewrite history by inserting passages into older dynastic chronicles, crafting persuasive new plotlines in which enemy and vassal kingdoms became eternal ethnic others. In the early nineteenth century, Bangkok’s artists and writers—including some commoners—began to treat the peoples of the world as a subject of study. They devised a conceptual vocabulary with which they could write about ethnic categorization abstractly, and they created ethnic typologies in prose, verse, and art. I argue that each of these innovations prefigured, in one way or another, Siam’s modern practices of ethnic identification. These innovations were not cooked up in isolation; rather, I show that they were inspired by the rhetorical trends and administrative practices of the diverse peoples who visited Siam from lands as distant as Lanka, China, Java, Persia, Europe, and America. At the same time, my work challenges paradigms of nineteenth-century intellectual change in which European modes of social organization simply and suddenly displaced static “traditional” ones. While modern visions of the Thai nation owe much to the intellectual frameworks of European colonial rule, my work reveals that they were also built on early modern innovations in the mobilization of ethnic labels. Indeed, these innovations primed Siam’s elites to recognize modern Western concepts of “nation,” “ethnicity,” and “race” in the late nineteenth century, and translate them in potent, familiar ways.