Ancient lakes contain unique and very vulnerable fauna. Determining and understanding the origin of such biodiversity is a key factor in promoting conservation and management actions in some of the most singular ecosystems on the planet. Lake Ohrid in the Balkans is known as a natural laboratory for speciation, containing a high number of endemic species. However, the identity and origin of the planktonic cladoceran Diaphanosoma is uncertain. Representatives of the genus were long considered to have invaded the lake, but recent morphological studies have suggested that they belonged to the endemic taxon in the Balkans, D. macedonicum. Here, phylogenetic methods based on two mitochondrial gene fragments (COI and 16S) were used to identify Diaphanosoma specimens from the ancient Lake Ohrid and Lake Prespa in the Balkans and compare them with other species in Europe, including those living in nearby water bodies. Molecular evidence showed that D. macedonicum was constrained to the ancient lakes Ohrid, Prespa, and Mikri Prespa, which suggests reproductive isolation within the lakes. Phylogenetic analyses supported previous morphological assessments and situated D. macedonicum within the D. mongolianum species group, which contains three sibling species (D. mongolianum, D. lacustris, and D. macedonicum). Nuclear markers are needed to study intraspecific gene flow in these organisms and discard a potential formation of hybrids.