The climate response to a massive release of methane from gas hydrates is simulated in two 2500-year-long numerical experiments performed with a three-dimensional, global coupled atmosphere-sea ice-ocean model of intermediate complexity. Two different equilibrium states were used as reference climates; the first state with preindustrial forcing conditions and the second state with a four times higher atmospheric CO2 concentration. These climates were perturbed by prescribing a methane emission scenario equivalent to that computed for the Paleocene/Eocene thermal maximum (PETM; ~55.5 Ma), involving a sudden release of 1500 Gt of carbon into the atmosphere in 1000 years. In both cases, this produced rapid atmospheric warming (up to 10¿C at high latitudes) and a reorganization of the global overturning ocean circulation. In the ocean, maximum warming (2--4¿C) occurred at intermediate depths where methane hydrates are stored in the upper slope sediments, suggesting that further hydrate instability could result from the prescribed scenario.