Modeling of gas hydrate formation below the seafloor requires accurate estimates of the methane solubility in marine sediments. Although the methane solubility is relatively well-established, a practical method for calculating solubilities under marine conditions is currently unavailable in the literature. We present a method of predicting the solubility in a marine setting for a given water depth, seafloor temperature and geothermal gradient. In this method, pressure and temperature conditions at the base of the hydrate stability zone (HSZ) are determined by finding the intersection of the local P, T conditions with experimentally determined conditions for three-phase equilibrium between water, hydrate and free gas. Phase equilibrium calculations of Zatsepina and Buffett [Zatsepina, O.Y., Buffett, B.A. (1997) Geophys. Res. Lett. 24, 1567^1570] are used to predict the methane solubility at the base of the HSZ, and simple parametric models are used to extend the solubility into the HSZ. The depth dependence of the methane solubility is computed at four known hydrate locations to provide constraints on the primary source of methane at these locales.