We investigated hydrothermal gas venting associated with a coastal fault zone along the western margin of Bahía Concepción, B.C.S., México. Copious discharge of geothermal liquid (≈ 90 °C) and gas is occurring in the intertidal and shallow subtidal zones (to a depth of 13 m) through soft sediments and fractures in rocks along a ∼750 m linear trend generally sub-parallel to an onshore fault near Punta Santa Barbara. Hydrothermal activity shows negative correlation with tidal height; temperatures in the area of hydrothermal activity were up to 11.3 °C higher at low tide than at high tide (measured tidal range ≈ 120 cm). Gas samples were collected using SCUBA and analyzed for chemical composition and stable isotope values. The main components of the gas are N₂ (≈ 53%; 534 mmol/mol), CO₂ (≈ 43%; 435 mmol/mol), and CH₄ (≈ 2.2%; 22 mmol/mol). The δ¹³C values of the CH₄ (mean = − 34.3‰), and the ratios of CH₄ to C₂H₆(mean = 89), indicate that the gas is thermogenic in origin. The carbon stable isotopes and the δ¹⁵N of the N₂ in the gas (mean = 1.7‰) suggest it may be partially derived from the thermal alteration of algal material in immature sedimentary organic matter. The He isotope ratios (³He / ⁴He = 1.32 R_A) indicate a significant mantle component (16.3%) in the gas. Here, we suggest the name El Requesón fault zone for the faults that likely formed as a result of extension in the region during the late Miocene, and are currently serving as conduits for the observed hydrothermal activity.