Magnesium-based bio-absorbable implants (MgBIs) are emerging as a promising alternative to conventional implants in current clinical practice. In this context, surgical planning and management could be significantly enhanced through advanced multiscale and multiphysics numerical models. As a first step in this direction, a phase-field approach is herein presented to model the degradation kinetics of MgBIs. The governing equations are consistently derived by minimizing the free energy, and are numerically solved using a finite-element formulation implemented with in-house codes. The model has been successfully validated by reproducing the dissolution of a cylindrical electrode in an electrolyte solution, and then applied to simulate the degradation of a magnesium-based screw. Available analytical results and experimental observations are well reproduced, highlighting the model’s soundness and effectiveness.