A gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data. From the depth of the reflectors, heat-flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments. This method is applied to the data in the Nankai Trough, around Central America, and along the Blake Outer Ridge. The estimated values, consistent with those measured by conventional means, suggest that derivation of heat flow by this method is feasible. On the landward slope of the Nankai Trough we find that heat flow increases downslope toward the floor of the trough.