The fouling barnacle Amphibalanus improvisus is a cosmopolitan biomonitor of trace metal bioavailability in coastal waters of reduced salinity such as estuaries or the Baltic Sea. Bioavailability of Ag, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn to A. improvisus was investigated in July 2014 at 15 sites along the Iranian coast of the understudied, yet biologically unique, Caspian Sea. In general, the bioavailability of these toxic metals in the water column, as biomonitored by the barnacle, was within the typical ranges expected from data available in the literature. Thus, there were no major environmental concerns raised. Nevertheless increased Cu and Zn were locally bioavailable at one site, perhaps associated with shrimp farming activities, and of Mn at another, probably from an industrial source. It is vital that the degree of contamination of the largest continental body of water in the world, particularly a water body with a unique biology is monitored.