We document the occurrence of the rayed creekshell (Anodontoides radiatus Conrad), a freshwater mussel (Unionidae), at eight sites in the upper Yazoo River drainage (lower Mississippi River Basin) in northern Mississippi. Previously, A. radiatus was thought to be restricted to Gulf Coast drainages as far west only as the Tickfaw River system (Lake Pontchartrain Basin), Louisiana. The eight populations reported herein represent the only known occurrences of this species in the Mississippi River Basin. This distributional pattern, along with distributions of other aquatic organisms, suggests that headwater stream capture events occurred historically between lower Mississippi River Basin tributaries and the upper Tombigbee River drainage (Mobile Basin). Because A. radiatus is a rare species, considered imperiled throughout its range, the discovery of eight additional populations and the extension of its known range are of significance to its conservation.