Gonimosobdella klemmi n. gen., n. sp. parasitic on the caudal, pectoral, and anal fins of the stoneroller, Campostoma anomalum pullum, and largescale stoneroller, Campostoma oligolepis, in Arkansas and Missouri, U.S.A., and the redfin shiner, Lythrurus umbratilis, and red shiner, Cyprinella lutrensis, in Illinois, U.S.A, is described. Gonimosobdella n. gen. is characterized by having a wide, flat body, 2 pairs of eyes on the oral sucker, 3(5) annuli per segment, 11 pairs of pulsatile vesicles, segmental connections between the coelomic sinuses and the pulsatile vesicles, fused postceca with fenestrae, conducting tissue, accessory gland cells on the atrial cornua, greater than 6 pairs of testisacs, long ovisacs, but lacks mycetomes and intersegmental connections of the coelomic system. Gonimosobdella klemmi n. sp. displays the characteristics of the genus, is generally cream colored with transverse bands of black stellate chromatophores and small orange-pigment flecks, and has a mouthpore in the posterior half of the oral sucker, postceca fused with 4 fenestrae, 13 pairs of testisacs, and conducting tissue in the form of a single strand originating from the bursa that splits into 2 strands around the common oviduct and then anastomoses into a single mass posterior to the female pore.