Late glacial sea level curves located in the Cascadia subduction zone (CSZ) fore arc in southwestern British Columbia show that glacial isostatic adjustment (GIA) was rapid when the Cordilleran Ice Sheet collapsed in the late Pleistocene. GIA modeling with a linear Maxwell rheology indicates that the observations can be equally well fit across a wide range of asthenospheric thicknesses, provided that the asthenospheric viscosity is varied from 3 × 1018 Pa s for a thin (140 km) asthenosphere to 4 × 10 19 Pa s for a thick (380 km) asthenosphere. Present-day vertical crustal motion predicted by the GIA models shows rates of a few tenths of a millimeter per year, consistent with previous analyses. The model viscosities largely pertain to the viscosity of the oceanic mantle beneath the subducting Juan de Fuca slab but include a contribution from the mantle wedge above the slab. For comparison, effective viscosities for the upper mantle due to the tectonic regime (subduction) were computed using the strain rates and temperatures of an independent geodynamic model of the CSZ with a wet olivine power law rheology. The effective viscosities agree well with GIA model viscosities of 1019 Pa s or less, corresponding to an asthenosphere of 100 or 200 km thickness. The agreement suggests a significant role for power law flow in the GIA response. Regardless of the microphysical mechanisms responsible for the GIA response, the viscosity values inferred from GIA can be applied to studies of the megathrust earthquake cycle because both processes take place on comparable time scales.