The last checklist of the water mites of the Balkan countries published in 2010 by Pešić et al. is updated to November 2017. This supplement includes new records of water mite species from the Balkan countries (Croatia, Bosnia & Herzegovina, Montenegro, Albania, Macedonia, Serbia, Bulgaria and Greece) published after 2010, as well as unpublished records based on material collected from Croatia, Montenegro, Albania, Macedonia, and Greece. Numerous new records for the national faunas, including one species new for the Mediterranean region (Arrenurus stjordalensis Thor, 1899), are reported and one species new to science (Trichothyas jadrankae Pešić sp. nov.) is described. With these additions, a total number of 390 water mite species and subspecies from 34 families and 77 genera is now recorded from the Balkan peninsula (including the Greek Islands).