The Caspian Sea is the largest closed lake in the world located in one of the most strategic energy production areas in the world. This sea is of great importance for the countries of the region in terms of management and legal issues due to its vast oil and gas resources, communication and commercial water lines, marine products, as well as environmental conditions, ecological balance, preservation of the environment and creation of beautiful tourist and natural landscapes in surrounding regions. Since the collapse of the Soviet Union, the four Treaties of the years 1921, 1931, 1935 and 1940 had established a solid legal framework between Iran and the Soviet Union, based on principles such as equality, monopoly, non-military identity of the sea, cooperation and ... However, after the collapse of the Soviet Union and the emergence of the issue of succession in that country and the formation of new states of Turkmenistan, Kazakhstan and Azerbaijan, legal problems have occurred and the policy of some of these newly independent states regarding the sea have raised doubts about the legal status of the Caspian Sea which can bring about undesirable political and economic consequences.
 The author, after referring to the legal status of this sea, has analyzed the problems created in its legal regime after the collapse of the Soviet Union using analytical and documentary methods, with conclusion that the existence of a valid, clear and firm legal regime based on previous (existing) treaties, while resolving the existing problems, will strengthen the sovereignty of Iran over the part of the sea in its vicinity.