The flow of gas through shallow marine sediments is an important component of the global carbon cycle and affects methane release to the ocean and atmosphere as well as submarine slope stability. Seafloor methane venting is often linked to dissociating hydrates or gas migration from a deep source, and subsurface evidence of gas-driven tensile fracturing is abundant. However, the physical links among hydrate dissociation, gas flow, and fracturing has not been rigorously investigated. We used mercury intrusion data to model the capillary drainage curves of shallow marine muds as a function of clay content and porosity. We combined these with estimates of in situ tensile strength to determine the critical gas saturation at which the pressure of the gas phase would exceed the pressure required to generate tensile fractures. Our work demonstrates that tensile fracturing is more likely as clay content increases due to decreased pore sizes and increased capillary pressure, but tends to be restricted to the shallowest portion of the sediment column (<130 m below seafloor) except when the clay-sized fraction exceeds 50%. Dissociating hydrate may supply sufficient quantities of gas to cause fracturing, but this is only likely near the updip limit of the hydrate stability zone, where release of methane bubbles from discrete vents is to be expected due to the combination of weak sediments and significant gas expansion. Gas-driven tensile fracturing is probably a common occurrence near the seafloor, does not require much gas, and is not necessarily an indication of hydrate dissociation.