The 2011 Tohoku-Oki earthquake caused significant post-seismic (PS) deformations in a broad region in East Asia including the southern Korean Peninsula (SKP). In this study, we aim to better characterize the PS deformations in the SKP and understand the underlying physical mechanisms for them using linear Maxwell viscoelastic numerical models. We computed the annual mean velocities by applying the extended linear trend modelling technique to the publicly available Global Navigation Satellite System data. We then derived yearly PS deformation rates for the first five years after the 2011 Tohoku-Oki earthquake using a least-square collocation method. The PS deformation rates were characterized by NE–SW compression and volumetric contraction in years 1 and 2; and by NNW–SSE extension and low rates of dilation in later years. The areas of relatively large PS maximum shear strain rates were in the central SKP in year 1 but shifted to the eastern and southeastern SKP in the following years. We estimated the true post-seismic (TPS) deformation rates by assuming the year 5 velocity gradients are the interseismic components and subtracting them from the previous years’ velocity gradients. The TPS deformation rates showed NNE–SSW compression and volumetric contraction across the SKP for all five years while their magnitudes monotonically decreased over time. The TPS volumetric strain rates and maximum shear strain rates exhibited clear spatial correlations with the known tectonic units in the SKP, of which boundaries run NNE–SSW. Our numerical simulations computed linear Maxwell viscoelastic responses of lithosphere and asthenosphere to a static slip distribution model for the 2011 Tohoku-Oki earthquake. Lithospheric thickness variations from LITHO1.0 were imposed. When compared with a model with a uniform lithospheric thickness of 60 km, the model with the non-uniform lithospheric thickness showed relatively large shear strain rates in thinner lithosphere. The misfit between the TPS deformation rates and our numerical simulations showed a greater sensitivity to asthenospheric viscosity than to asthenospheric thickness. The misfit was consistently small for the asthenosheric viscosity of around 10$^{19}$ Pa$\cdot$s mostly regardless of the bottom depths of asthenosphere between 200 and 400 km. Our findings, along with the tectonic history of the SKP, suggest that the regions of elevated TPS maximum shear strain rates might have thinner or weakened lithosphere, which can be one of the consequences of the pervasive granitic intrusions in those regions.