Stenelmis morsei, a new species of riffle beetle, is described from the Upper Three Runs Creek drainage of Aiken County, South Carolina. It closely resembles Stenelmis lateralis Sanderson of the crenata-group in elytral pattern but differs in the distinctly smaller size (2.1-2.3 mm long) and in the width of the pronotum being only slightly narrower than the width of the elytra. Outline drawings of the elytra, pronotum, and male genitalia are given to show differences between the two species along with an interpolation of the new species into existing keys.