Prior to this study, undocumented morphological variation in Lythrurus umbratilis (Redfin Shiner) has impaired identification of many samples of eastern populations of the species. Meristics, morphometrics, tuberculation, pigmentation, and nuptial male coloration of over 700 specimens of L. umbratilis were examined in order to assess patterns of geographic variation and species limits in the Ohio River basin. Principle component and spatial autocorrelation analyses of these data demonstrate that morphological variation in L. umbratilis is clinal along most of the length of the Ohio River basin. Specimens from eastern populations have less black in the dorsal fin of breeding males, have lower mean scale counts, and are more slender than western populations. The cline does not extend into central and northern Ohio, as populations have relatively high meristic counts and more robust males. These analyses suggest that recognition of an additional species in the Ohio River basin is not warranted at this time. The cline may reflect the influence of drainage evolution of the region, or even past gene flow with Lythrurus fasciolaris (Scarlet Shiner), a closely related species with a distribution parapatric to that of L. umbratilis.