In subduction zones, the size of the seismogenic zone that ruptures during a great thrust earthquake may be thermally controlled. We have constructed finite element thermal models of six transects across three different subduction zones in order to determine the temperature distribution along the plate interface and to predict the size of the seismogenic zone. These models incorporate the complex plate geometries (variable dip downsection) necessary to model a flat slab subduction style. We focus on the rupture zones of the great earthquakes of Nankai (SW Japan) 1946 M8.3 and Alaska 1964 M9.2 as well as on the Cascadia margin. Subduction zone segments with moderate to steep dips exhibit rupture zones of 100–150 km downdip width, consistent with earlier elastic dislocation models and thermal models. For shallow dipping flat slab segments, our models predict larger locked zones, of 150–250 km width, in good agreement with aftershock and geodetic studies. The wider seismogenic zone predicted for flat slab segments results from an uncommonly wide, cold, forearc region, as corroborated by surface heat flow observations. A global analysis of great M > 8 interplate earthquakes of the 20th century reveals that more than a third of these events occurred in flat slab segments, whereas these segments represent only 10% of modern convergent margins. This implies substantially higher interplate coupling for flat slab segments, likely due to the increased downdip extent of the seismogenic zone, and suggests that the seismic risk near such regions may be higher than previously thought.