In the Cascadia subduction zone, the extent of the seismogenic portion of the plate interface is poorly resolved by seismicity due to the lack of a large megathrust event during the instrumental record. Therefore, fault zone temperatures (∼150 to 350 °C) are used to estimate the limits of the seismogenic zone. Previous thermal models for the Cascadia margin estimated that 350 °C on the plate boundary occurs ∼40–70 km offshore. In contrast, models of interseismic deformation have been interpreted to indicate a seismogenic zone extending landward of the coastline to the updip edge of a region of episodic tremor and slip (ETS). We examined Cascadia subduction zone temperatures with thermal models that include the effects of fluid circulation in an ocean crust aquifer. Fluid circulation cools the subduction zone and widens the thermally defined seismogenic zone by shifting the 350 °C isotherm at the plate boundary ∼30–55 km landward relative to results from simulations without fluid flow. Our thermal models indicate a 60–80-km-wide transition zone between 350 °C on the fault and the updip edge of ETS. Under British Columbia (Canada), Washington, and Oregon (United States), ETS occurs at ∼410–550 °C. The location of the basalt-to-eclogite transition in the subducting crust provides an important constraint on the thermal models because hydrothermal circulation in the ocean crust aquifer produces only small surface heat flux anomalies on this margin with thick sediment in the trench and on the incoming plate.