Marine environment of the Caspian Sea, due to extensive human exploitation and discharge of large magnitudes of urban, industrial and agricultural waste, is under extensive pressure and human wastes threaten marine life and ecosystem of the sea. Therefore, there is a substantial need for a good understanding of physical and chemical characteristics of the Caspian waters. However, the data available in this regard, in particular in the Iranian coastal waters, are very limited. This paper presents distribution and seasonal variations of pH over the southern Caspian Shelf adjacent to Iran. The pH data were collected down to 200m depth in two areas in east (off Babolsar in Mazandaran) and west (off Kiyashahr in Gilan) of the southern coast of the Caspian Sea. In summer, when the surface water temperature was highest and the strongest seasonal thermocline existed, the maximum value for pH was observed. In this season, pH had a value of 8.35-8.4 at the water surface and reduced to 8.2 at 80-m level and deeper waters. In autumn, when the surface layer was deepening and seasonal thermocline was under destruction, the pH was less than summer and was mainly 8.25 at the water surface outside of the shelf and 8.3-8.35 in the shelf and gradually reduced with depth. In early spring, during the process of the formation of the new seasonal thermocline, low temperature and maximum local river inflow, pH at the water surface was mainly 8.25 and reduced to 8.2 at 30-m level. The results showed seasonal variations of pH in the southern Caspian Shelf and the coastal waters adjacent to Iran as a function of seasonal variations in water temperature, characteristics of thermocline, local rivers chemical characteristics and discharges, and production or destruction processes.