The diversity, distribution, and checklist of diaptomid copepods from various freshwater ecosystems throughout Thailand are presented, based on data from our biodiversity projects during 1993–2019 and literature reviews. Thailand has one of the most diversified diaptomid fauna in the world, with 42 species identified from 2150 localities (4962 samples). Mongolodiaptomus has the highest species richness with ten species, followed by Tropodiaptomus with seven species. Among these, eight taxa (Arctodiaptomus sp., Mongolodiaptomus pectinidactylus, Neodiaptomus meggitti, Tropodiaptomus hebereri, T. lanaonus, T. oryzanus, T. ruttneri, and Paradiaptomus greeni) are new to the fauna of Thailand. The rare P. greeni, which predominantly occurs in Africa, is also a new record for Southeast Asia. The most frequently encountered species were Mongolodiaptomus botulifer, Phyllodiaptomus praedictus, M. calcarus, M. dumonti, M. malaindosinensis, Vietodiaptomus blachei, Phyllodiaptomus christineae, Eodiaptomus sanoamuangae, Neodiaptomus yangtsekiangensis, E. draconisignivomi, T. vicinus, and Heliodiaptomus elegans. Twelve species appear to be endemic to Thailand, and eight species occur only in the countries belonging to the lower Mekong River Basin (Thailand, Cambodia, Laos, and Vietnam). The uniqueness of the Thai diaptomids is a high degree of co-occurrence of four to seven species in the same localities. Comments on the taxonomic status of the species recorded are provided. In addition, the taxonomic validity of Mongolodiaptomus malaindosinensis is discussed.
