KNOWLEDGE of the viscosity structure of the Earth's mantle is important for constraining models of mantle convection and isostatic rebound. Here we show that seismicity around the margins of deglaciated areas provides a constraint on the viscosity of the lower mantle, in addition to those previously proposed1,2. Calculations using a spherical, viscoelastic Earth model show that the present-day magnitude of the stress fields induced in the lithosphere beneath the (now-disappeared) Laurentide and Fennoscandian ice sheets is very sensitive to the value of the lower-mantle viscosity. Stresses of ∼100 bar, sufficient to cause seismicity, can still remain in the lithosphere for lower-mantle viscosities greater than ∼1022 Pa s; for lower-mantle viscosities of ∼1021 Pa s, only a few tens of bars of stress persist in the lithosphere today. This influence of lower-mantle viscosity on the state of stress in the lithosphere also has implications for the migration of stress from earthquakes, and hence for earthquake recurrence times.