A new approach for measuring flow stresses near a spinning friction stir welding (FSW) tool is evaluated on AA 6061-T6 plate. The test consists of plunging a cylindrical tool with a flat face into the plate at different rotational speeds, using a variety of constant vertical loads. A viscosity-based model of the shear layer created under the tool is employed to estimate local flow stresses. The flow stresses measured by this approach exhibited an inverse relationship with temperature and a positive dependence on the pressure imposed by the spinning flat-faced tool. Compared to hot compression and hot torsion results, estimated flow stress levels in high-pressure shear were lower by 20–68 %, for similar temperatures and strain rates, owing to grain refinement induced by continuous dynamic recrystallization. This high-pressure shear approach could be used to characterize material behavior near a rapidly spinning FSW tool, leading to improved process model predictions.