Study of a large collection of pinnotherid crabs collected primarily from Singapore and Peninsular Malaysia revealed substantially more variation in the carapace shape, third maxilliped, and other taxonomic characters than previously acknowledged. While carapace shape and form, structures of the epistome (especially the posterior margin), antennal article 1, eyes (especially the cornea), third maxilliped, male and female chelae, ambulatory pereopods, male pleon, as well as features of the male first and second gonopods are useful taxonomic characters, the degree of variation must be better accounted for so that accurate species and genus assignments can be made. The female-to-male ratio is also indicative for several species. Also significant is that the host of the pinnotherid species is taxonomically correlated and many taxa are quite specific to the mollusc genus, family, or superfamily. As a result of this reappraisal of the taxonomic characters used, 14 names are synonymised under five taxa. The following new synonymies are recognised: Pinnotheres hirsutissima Johnson, 1963, a nomen nudum, under Holotheres semperi (Bürger, 1895); P. rhombifer Bürger, 1895, P. latissimus Bürger, 1895, P. arcophilus Bürger, 1895, P. paralatissimus Dai & Song, 1986, and Arcotheres guinotae Campos, 2001, under Arcotheres palaensis (Bürger, 1895); P. modiolicola Bürger, 1895, under Arcotheres similis (Bürger, 1895); P. winckworthi Gordon, 1936, P. vicajii Chhapgar, 1957, P. casta Antony & Kutyamma, 1971, and P. obscuridentata Dai & Song, 1986, under Arcotheres exiguus (Bürger, 1895); and P. latus Bürger, 1895, P. borradailei Nobili, 1906, and P. ridgewayi Southwell, 1911, under P. globosum Hombron & Jacquinot, 1846, which is referred to a new genus, Magnotheres. In addition, we establish a new genus, Plenotheres, for an unusual member of Arcotheres Manning, 1993, which lives in mangrove clams of the families Cyrenidae Gray, 1840, and Glauconomidae Gray, 1853. The poorly known Japanese species, Pinnotheres corbiculae Sakai, 1939, which lives in corbiculid clams and superficially resembles Plenotheres coarctatus, new combination, is redescribed from a male specimen and here referred to Nepinnotheres Manning, 1993. Likewise, Pinnotheres tsingtaoensis Shen, 1932, is also referred to Nepinnotheres.