Species diversity of ostracods in the floodplain of the Chi River basin, Mahasarakham Province, Thailand was examined in this study. Twenty-two samples were collected in the cool season (February 2010). Twenty-six species belonging to 18 genera were recorded, 3 of which are new to the Oriental region (Fabaeformiscandona japonica (OKUBO, 1990), Ilyocypris monstrifica (NORMAN, 1862) and Limnocythere stationis VÁVRA, 1891) and 1 (Hemicypris ovata SARS, 1903) is a new record for Thailand. The most common species were Strandesia kraepelini (G.W. MÜLLER, 1906) and Physocypria crenulata (SARS, 1903) occurring in 77% and 64% of the sampled localities, respectively. Additionally, males of Tanycypris siamensis SAVATENALINTON & MARTENS, 2009 were found for the first time. A description of T. siamensis males, focusing on the male reproductive organs, is here provided. This discovery shows that the morphology of the hemipenis and the Zenker organs of T. siamensis is similar to that of species in the genus Cypricercus. Additional comments on taxonomy and ecology of several taxa are also given.