Megasolena dongzhaiensis n. sp. was collected from the intestine of Scatophagus argus (Linnaeus) (Perciformes: Scatophagidae) from the Dongzhai Bay (110°32′–37′E, 19°51′–20°1′N), Hainan Province, China. It resembles Megasolena acanthuri Machida and Uchida, 1991 in having larger body size, but it differs from the latter species in having an oral sucker that is larger, instead of smaller, than the acetabulum (sucker length ratio 1:0.456–0.494, and width ratio 1:0.61–0.65 in M. dongzhaiensis n. sp., as opposed to 1:1.3–1.7 in M. acanthuri). Moreover, the cuticle is spinose rather than aspinose; there is a band of circular muscle in the pharynx, and its eggs are smaller instead of larger (0.062–0.068 × 0.036–0.039 in M. dongzhaiensis n. sp., compared with 0.087–0.103 × 0.058–0.072 in M. acanthuri). Finally, the intestinal bifurcation is anterior, instead of dorsal, to the acetabulum. It resembles other species of Megasolena Linton, 1910 in having oral sucker larger than the acetabulum, and in having a circular muscle band in the oral sucker and pharynx, but it differs in having a larger body and smaller eggs. This is the first record of a Megasolena species from ray-finned fishes as well as in China.