Abstract: The subject of the study is the process of development of environmental protection activities of post-Soviet Russia, Kazakhstan, Turkmenistan, Azerbaijan, as well as Iran in the Caspian Sea. The study used such methods of historical research as retrospective analysis, comparative analysis, document analysis, periodization method, systematic approach. The author considers such aspects of the topic as the problem of periodization of complex social processes, the actual periodization of the development of joint environmental activities of the Caspian states, the main characteristics of the stages of development of international relations in the field of environmental activities in the Caspian Sea. The author pays special attention to the problem of awareness by the "Caspian five" of the ecological integrity of the Caspian Sea and the need to develop collective solutions to environmental problems and the protection of the natural diversity of the sea. В  As a result of the analysis, the author's periodization of the development of environmental activities in the Caspian Sea in the post-Soviet period of the history of the region and the characteristics of the selected stages are proposed, which is the novelty of the study and the author's special contribution to the study of the topic. In total, the author identifies three stages that differ qualitatively from each other (the 1990s, 2000s and 2010s). The periodization is based on a qualitative assessment of the development of relations between the national states of the Caspian region within the framework of the regional subsystem of international relations, characterized as the "Caspian node", which distinguishes it from other well-known approaches to periodization and characterizes as promising.