A grey seal (Halichoerus grypus), representative of the North Atlantic species, has been recorded in the north-east Black Sea. It is the first documented case of successful long-term survival of an exotic pinniped. We have been receiving data about regular sightings of the seal identified as the observed individual since 2001. It is a 160–170 cm long adult female. The seal used an underwater cave as a shelter. The most likely way of introduction of the grey seal to the Black Sea is escape from captivity. According to available data (body size and moulting seasonality), we tentatively identify it as a representative of the Baltic subspecies. The biotope requirements of the grey seal and monk seal are similar: both species use coastal karst caves and grottos. In addition, the seal's presence in this region is a marker of the lack of anthropogenic disturbance. Thus, the survival of a seal in this region indicates the possibility of successful re-colonization of the Black Sea by monk seals.