Modeling of postseismic deformation following great earthquakes has revealed the viscous structure of the mantle and the frictional properties of the fault interface. However, for giant megathrust events, viscoelastic flow and afterslip mechanically interplay with each other during the postseismic period. We explore the role of afterslip and viscoelastic relaxation and their interaction in the aftermath of the 2011 Mw (moment magnitude) 9.0 Tohoku earthquake based on a detailed model analysis of the postseismic deformation with laterally varying, experimentally constrained, rock rheology. Mechanical coupling between viscoelastic relaxation and afterslip notably modifies both the afterslip distribution and surface deformation. Thus, we highlight the importance of addressing mechanical coupling for long-term studies of postseismic relaxation, especially in the context of the geodynamics of the Japan trench across the seismic cycle.