An annotated review of the Mesoveliidae (Hemiptera: Heteroptera: Gerromorpha) of the south-east European countries is provided. Mesovelia thermalis Horváth, 1915 is recorded for the first time from Romania outside of its type locality and for the first time from Bulgaria, Serbia, and Croatia. Therefore, its European geographical range is two times larger than previously known. Mesovelia vittigera Horváth, 1895 is excluded from the faunas of Romania and Moldova based on proven or suspected misidentifications and new information is provided about the distribution of M. furcata Mulsant et Rey, 1852. We analysed the phylogenetic relationships of M. thermalis within the genus and further examined additional south-east European individuals of M. vittigera and M. furcata. The results revealed that M. thermalis is a possible sister species to M. furcata, and that the south-eastern individuals of both M. vittigera and M. furcata are closely related to their conspecifics. Molecular dating based on fossil calibration indicated that the crown age of Mesovelia Mulsant et Rey, 1852 is ca. 22.8 Myr and the last common ancestor of M. thermalis and M. furcata lived ca. 4.7 Myr ago. Finally, an illustrated identification key to the European species of Mesovelia is provided.