Ostreobdella californiana n. sp. is described from rockfishes in the genus Sebastes in public display tanks in the Steinhart Aquarium, California Academy of Sciences, San Francisco, California, U.S.A., and in the Bodega Marine Laboratory, University of California, Davis, Bodega Bay, California. New species justification was based on external morphology, histology, and molecular sequence data. Leeches are small, up to 12 mm total length. Oral sucker is larger than caudal sucker, with 1 pair of eyes on the oral sucker and 10 ocelli on the caudal sucker. Segmental ocelli occur both dorsally and ventrally on the urosome. Small papillae are present on urosome segments, but pulsatile vesicles are absent. There are 6 pairs of testisacs. The male reproductive system has a muscular organ replacing the typical atrium and a complex system of conducting/vector tissue. Mycetomes are present, and postceca are fused with fenestrae at each ganglion.