H. malcolmi is distinguished from congeners by the following combination of characters: 12-17 (usually 14-15) rakers on first gill arch; usually 16 circumpeduncular scale rows; less than 30 lateral line scales; some branched lateral line canals, but branching not extensive, median portion of dorsal-fin dark.

Known from various parts of the middle and upper Mekong river basins with record existing from the Nam Tha, Nam Ou, Nam Khan, Nam Lik, Nam Ngum and Nam Mang tributary systems in Laos, the Nam Noeua (a tributary of Nam Ou) in Vietnam, and the Mae Nam Kok in Thailand.

Occurrences in the Chao Phraya drainage in central Thailand most likely refer to the congener R. chiengmaiensis.