Most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment, the décollement, and wedge likely transports only a minor amount of heat. We model coupled fluid and heat transport in a subduction zone and show that hydrothermal circulation in subducting basaltic basement rocks can have a great influence on subduction zone temperatures. Fractured basaltic basement has permeability several orders of magnitude higher than a typical décollement, allowing fluid circulation to redistribute and extract heat from a subduction zone. We simulate systems with upper basaltic basement permeability ranging from 10−13 to 10−10 m2. In addition, we incorporate the effect of permeability reduction within the basaltic basement as it is subducted. The models with fluid transport show suppressed temperatures along the subducting slab relative to models with no fluid transport. With continuous sediment cover, heat is extracted from under the margin wedge to the trench. In models where faulted ocean crust exposes high-permeability basement to the ocean floor, cooling from ocean bottom water results in highly suppressed heat flow relative to conductive models. Hydrothermally cooled ocean crust also acts to slow thermally controlled diagenetic reaction progress within subducting sediment.