Two new and 1 previously described species of Thylacicleidus Wheeler and Klassen, 1988, are reported from 2 species of brackish to freshwater puffers (Tetraodontidae) from Thailand. The generic diagnosis of Thylacicleidus is emended to incorporate the following characters: presence of a dextral muscular vagina, a vas deferens looping the left intestinal cecum, 2 prostatic reservoirs, a sclerotized tubular copulum without an accessory piece, and dorsal anchors with reduced outer root. The type species, Thylacicleidus serendipitus, is redescribed based on new information obtained from specimens found on the gills of the type host, Tetraodon nigroviridis Marion de Procé, 1822 (spotted green puffer). Two new species, Thylacicleidus latus n. sp. and Thylacicleidus brunensis n. sp., are described from the gills of T. nigroviridis and Tetraodon biocellatus Tirant, 1885 (eyespot puffer), respectively. The occurrence of common generic characters in species of Thylacicleidus and Haliotrema Johnston and Tiegs, 1922, another dactylogyrid genus recorded from tetraodontiform hosts, signifies the close relationship between the 2 genera and raises the question of synonymy. Nevertheless, because of morphological variability within Haliotrema spp., extremely wide host spectrum and the probable polyphyly of the genus, it is suggested that Thylacicleidus is a valid genus.