This article explores the history of the little-known polygamy exclusion in the 1891 Immigration Act. Tracing the development of this polygamy restriction in the late nineteenth century, the article shows how anxieties about the European migration of Mormon converts shaped immigration legislation, which was then almost immediately redirected to regulate the migration of Turkish immigrants by the early 1900s. Racialized as “American Mohammedans,” Mormons came to embody a foreign, “uncivilized,” and semi-imperial threat for many nineteenth-century Americans. As the persecution of Mormons receded in the 1890s, however, the increasing visibility of Muslim immigrants from the Ottoman Empire fed new fears about religious difference, racial inferiority, and sexual and marital deviance. US immigration officials addressed such concerns by attempting to apply the polygamy exclusion on Turkish immigrants, but soon found themselves at the center of diplomatic tensions between US and Ottoman officials. Analyzing the imperial negotiations at play as US immigration law ran up against the power of the Ottoman Empire, the article brings deeper attention to the ways in which the policing of polygamy and “proper” marital formations were ultimately shaped by anxieties over race, religion, and empire.