In the context of a complete revision of the family Onchidiidae, two new species of Melayonchis Dayrat & Goulding in Dayrat et al., 2017, are described from western Peninsular Malaysia, in the Strait of Malacca: Melayonchis eberlyi Dayrat & Goulding, new species, and Melayonchis tillieri Dayrat, new species. The genus Melayonchis includes a total of six species, the four species originally described in 2017 and the two new species described here. The delineation of all six species is supported by comparative anatomy as well as mitochondrial (COI and 16S) and nuclear (ITS2 and 28S) DNA sequences. Both new species are identifiable in the field using external traits. New records are provided for Melayonchis eloisae Dayrat in Dayrat et al., 2017 in eastern Sumatra, and for Melayonchis siongkiati Dayrat & Goulding in Dayrat et al., 2017, in the Northern Territory, Australia. The isolated presence of Melayonchis in northern Australia suggests that the geographical distribution of Melayonchis is more extended than what was originally discovered (Andaman Sea, Strait of Malacca, and South China Sea). The diagnosis of the genus Melayonchis and the anatomical differences between Melayonchis species are updated. A noteworthy trait is the presence in M. tillieri of up to 420 radular lateral teeth in each half row (for a total of 840 lateral teeth per row), making it the onchidiid species with the highest number of teeth. Finally, differences between the intestinal types are clarified, most especially the types II and III, both found in Melayonchis.