Abstract <p>The results of complex oceanographic and gas-geochemical measurements in the La Perouse Strait are presented. The measurements revealed elevated methane concentrations in the water (up to 14 nM/L). The content of He (up to 11 ppm) and H₂ (up to 23 ppm) was determined in the water samples. The area of gas emission from the bottom source in the western part of the strait has been found. High turbidity values, as well as a decrease of salinity in the maximum methane concentration area suggest the presence of a gas-saturated fluid vent, which can be related to mud volcanism. Comparison of the free gas composition and isotope analysis showed the similarity between mud volcanoes in the southern Sakhalin Island and the northern Hokkaido Island. Probably, the chain of Hokkaido–Sakhalin mud volcanoes is associated with the meridional fault system in the Hokkaido–Sakhalin folded region.</p>