A key is given to 7 species, 6 of which occur in Malesia. Of each the basionyms and a restricted synonymy are given, besides notes on their distribution. Rotala diversifolia Koehne, hitherto only known from Thailand, appears to occur in several localities in Malesia. A new combination, R. catholica (Cham. & Schlechtend.) B. van Leeuwen, is proposed for the American R. dentifera (A. Gray) Koehne, introduced in Luzon.