A representative of the family Ectinosomatidae was discovered in a temporary pool in a cave in the Satun Province, southern Thailand. Based on the characteristics of the antennary exopod, armature of the female fifth leg, and configuration of the male sixth leg, a new species of the genus Rangabradya was identified, representing the first record of the genus in the continental waters of Thailand and in Southeast Asia. The new species can be distinguished from Rangabradya indica by the configurations of the fifth and the sixth legs in both sexes, the body ornamentation of the urosomite, and the armature of the mouthparts. These structures show a more primitive state in the new species. Accordingly, a new subgeneric rank in the genus Rangabradya, Siamorangabradya subgen. nov, was established to accommodate the Thai species and Rangabradya (Siamorangabradya) wongkamhaengae subgen. et sp. nov. was described. Also, the key to all 23 genera of the family was updated.