Euryhaline organisms are organisms that have a high tolerance range for salinity and can tolerate wide fluctuations in salinity. One of the habitats of eurrhinic organisms is the Caspian Sea. As the largest lake on Earth, this lake was a remnant of the ancient Paratethys Ocean during its formation and evolution. To investigate the evolution of the Caspian Lake and its relationship with the euryhaline microorganisms in it, two areas of the southern Caspian, the coasts of Khazarabad and Mahmudabad, were studied. The diatom species Actinocyclus alienus and Actinoptychus senarius, which live in the seas, were reported from its waters. Also, calcareous nanoplankton species that are indicative of seas with warm and salty waters, such as Calcidiscus cf.C. leptoporus, Sphenolithus cf. C. moriformis, Sphenolithus sp., and Nannolithes, were found in these waters. Given the closed nature of the Caspian basin, the high evaporation rate, and the limited freshwater input, the existence of these organisms can be considered an indication of their presence in the remaining ocean waters from the Late Miocene (Serravalian, about 11.6 million years ago) to the present day, and their presence is highly dependent on water salinity levels for their survival and life.