In this study, the influence of microstructural refinement induced by the high-pressure torsion (HPT) on the corrosion resistance of the Ti-45Nb (mass%) alloy was investigated. The alloy characteristics before and after the HPT deformation were analyzed by electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM), x-ray diffraction (XRD), and Vickers microhardness measurements, while the alloy corrosion behavior in simulated physiological conditions was examined by potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) analysis. Detailed microstructural analyses revealed that the HPT deformation led to significant grain refinement of the Ti-45Nb alloy exhibiting an ultra-fine grained (UFG) microstructure along with a substantial increase of hardness. Results also indicated that the grain refinement did not affect the alloy phase composition since β-Ti and Ti4Nb phases were present in the microstructure before and after the HPT deformation. Even though the Ti-45Nb alloy in both, coarse-grained (CG) and UFG, conditions shows high corrosion resistance in Ringer's solution at 37 °C, it was observed that the HPT treatment additionally improved the alloy corrosion properties. Namely, more rapid formation of the passivating layer with better barrier properties on the UFG alloy surface was recorded and resulted in better corrosion resistance of the alloy after HPT deformation. An increase of the grain contact area in the refined microstructure caused an increase of the diffusive transfer along the grain boundaries, accelerated the formation of a less defective protective barrier surface layer, and promoted the alloy surface passivation in the simulated physiological conditions.