Mymarothecium viatorum sp. n. (Dactylogyridae, Ancyrocephalinae) is described from the gills of Piaractus brachypomus (Teleostei, Serrasalmidae) captured in the warm water canal of a power plant of the city of Szczecin, Poland. The new species is differentiated from other species in the genus by presenting a copulatory ligament that articulates to the accessory piece subterminally (not terminally), posteromedial projection on the ventral and dorsal bars; further, superficial root of the dorsal anchor that bends and articulates to the dorsal bar and vagina slightly sclerotized. This is apparently the first species of an introduced Neotropical Monogenoidea collected from the environs of Central Europe.