The West Siberian Basin (WSB) is one of the largest intracontinental basins in the world with and area of roughly 3.5 million km2. It is closely associated with the Siberian flood basalts which underlay the sediments and extend on to the neighboring Siberian Craton. However, the mechanism which formed the basin is still uncertain. Rifts have been observed on seismic and magnetic surveys in the north of the basin, but the basin size far exceeds the extent of the rifts. The sediment thickness varies between the rifted areas and the wider basin. In the rifts well logs show ~ 8 km of sediments. In areas of the basin out with the rifts the average sedimentary thickness is ~ 4 km thick. The reverse is seen in the crustal thickness, where the moho beneath the rifts is at a depth of ~34 km and is deeper (~38 km) in the wider basin. The sedimentation in the rifts begins almost immediately after the eruption of the basalts 250 Ma and continues through to the present day. Outside the rifts the top of the basalt, where it is present, forms an erosional surface and sediment deposition doesnt begin for 50-70 Myrs with the first basin wide transgression seen at 164 Ma. We used a numerical 1D conductive heatflow model to calculate the isostacy and temperature change through a 170 km thick column of the crust, lithosphere and upper mantle. This was used to test whether the cooling of a hot layer at the base of the lithosphere could explain the basin formation. We found that cooling of a 1500C 50 km thick layer below a 50 km thick lithosphere fitted the subsidence curves from the within the rifts if a 34 km thick crust is used. It also fits those from the wider basin when the crustal thickness is increased to 38 km even showing the delayed onset of subsidence seen. The thickness and temperature of the hot layer and extent of thinning of the lithosphere match observations from present day plumes and numerical models of plumes. Therefore we propose that the WSB formed due the decay of a plume head at the base of the lithosphere which caused uplift and rifting of the crust and thinning of the lithosphere accompanied by eruption of the Siberian flood basalts. This is followed by subsidence as the plume head cools. The initial uplift outside the rift has to drop below sea-level before sedimentation is seen outside the rifts explaining the delay.