An expansion of a species range may happen unnoticed when an expanding species is similar in appearance to local species and distinguishable mainly by genetic methods. The morphological similarity of polyploid spined loaches of the hybrid complex Cobitis elongatoides-taenia-tanaitica with their diploid relatives makes their recent expansion in the rivers of Eastern Europe truly cryptic. We confirm this expansion and describe it in space and time by analyzing 20 museum collections from 1928 to 1985 and 92 recent sampling points from the Danube, Dniester, Southern Bug, Dnieper, Donets, and coastal rivers of the Sea of Azov. According to the museum collections, the numerical increase of polyploids relative to diploids occurred in the 1960-1980s that coincides with the time of intensive hydraulic construction when a series of reservoirs were created in the Middle Dnieper basin. Our sampling during 2000-2020 suggests that the trend toward the numerical increase of polyploids also continues in the twenty-first century. This expansion could have occurred under new ecological conditions, both due to the few clonal individuals that spread into the rivers of Eastern Europe as a result of postglacial recolonization, and due to invasions of polyploids from the Danube, Vistula, and Oder basins in the middle of the twentieth century. The expansion of polyploids is probably a consequence of their relatively high fitness in comparison with diploids in conditions of significant anthropogenization of river systems.