During surveys of the trichodinid parasites in mariculture beds off the coast of Shandong Province, China, four species of the genus Trichodina from the gills of marine molluscs were investigated and morphologically studied. Of these, three are described as new: T. ruditapicis n. sp. from Ruditapes philippinarum (Veneridae), T. scapharcae n. sp. from Scapharca subcrenata (Arcidae) and T. mactrae n. sp. from Mactra veneriformis (Mactridae). One little-known species, T. macomarum Raabe & Raabe, 1959, is redescribed from M. veneriformis. Taxonomic and morphometric data for these trichodinids based on wet silver nitrate and protargol-impregnated specimens are presented. For each of the new species, comparisons with closely related species are provided.