The Miami-Illinois Language reconstructs the language spoken by the Miami and the Illinois Native Americans. During the latter half of the seventeenth century both Native communities lived in the region to the south of Lake Michigan in present-day Illinois and Indiana. The French and Indian War, followed in the late eighteenth and early nineteenth centuries by massive influxes of white settlers into the Ohio River Valley, proved disastrous for both Native groups. Reduced in number by warfare and disease, the Illinois (now called the Peorias) along with half of the Miamis relocated first to Kansas and then to northeast Oklahoma, while the other half of the Miamis remained in northern Indiana.

The Miami and the Illinois Native Americans speak closely related dialects of a language of the Algonquian language family. Linguist David J. Costa reconstructs key elements of their language from available historical sources, close textual analysis of surviving stories, and comparison with related Algonquian languages. The result is the first overview of the Miami-Illinois language.

David J. Costa works as a professional linguist in Native language revitalization and lives in northern California.