Abstract: Large-scale changes in the ranges and abundance of a large number of species of both southern and northern origin have occurred in the Baltic region under the development of the warm phase of the climate in recent decades. Moreover, changes are far from being synchronous in different species of these groups. An increase in the population of the Caspian Tern Hydroprogne caspia and its active settlement in the northeast direction were observed in the first half of the 20th century. In recent decades, there has been, on the contrary, a general downward trend in the number. The article discusses in detail the history of the invasion of the species in the eastern part of the Gulf of Finland, the spatial distribution, and features of the habitat confinement of nesting birds. It is shown that, unlike other southern invaders of the region, the Caspian Tern has remained a rare species over the past 30–40 years with a tendency to strong population fluctuations. This is probably caused by the lack of a reserve of birds in the nearest breeding areas. It is shown that the Caspian Tern, which is a stenobiont species, usually nests on open sandy biotopes. Inhabiting the Gulf of Finland, it has mastered reproduction in a fundamentally different landscape, on selga massive-crystalline rocks.