Hydrothermal circulation within oceanic basement can have a profound influence on temperatures in the upper crust, including those close to the subduction thrust and in the overlying plate. Heat flow evidence for hydrothermal circulation in the volcanic basement of incoming plates includes: (1) values that are well below conductive predictions due to the advection of heat into the ocean, and (2) variability about conductive predictions that cannot be explained by variations in seafloor relief or thermal conductivity. In this review we summarize evidence for hydrothermal circulation in subducting oceanic basement from the Nankai, Costa Rica, south-central Chile, Haida Gwaii, and Cascadia margins and explore its influence on plate boundary temperatures. Models of these systems using a high Nusselt number proxy for hydrothermal circulation are used to illustrate the influence of this process on seafloor observations and thermal conditions at depth. We show that at these subduction zones, patterns of seafloor heat flow are best explained by thermal models that include the influence of hydrothermal circulation.