Biodegradable magnesium (Mg) alloys are promising materials for temporary biomedical implants due to their favorable mechanical properties and bioresorption capability. However, their structural integrity is severely compromised by stress corrosion cracking driven by corrosion-induced hydrogen uptake, which can lead to premature and brittle failure under physiological loading. Predictive assessment of such failure requires robust multiphysics computational frameworks capable of capturing the interaction between mechanical deformation, hydrogen transport, and fracture. In this work, a coupled chemo-mechanical phase-field framework is developed to simulate hydrogen-assisted crack initiation and propagation in biodegradable Mg implants. The formulation combines linear elastic deformation, stress-assisted hydrogen diffusion, and phase-field fracture within a unified variational setting. Hydrogen transport is governed by an extended diffusion equation incorporating hydrostatic stress–driven flux, enabling hydrogen accumulation in tensile stress regions ahead of crack tips. Hydrogen embrittlement is introduced through a concentration-dependent degradation of the critical energy release rate, representing hydrogen-enhanced decohesion in Mg alloys. Crack evolution is captured using a phase-field approach, allowing fracture to emerge naturally without explicit crack tracking. Numerical simulations are performed on a pre-cracked Mg specimen subjected to Mode-I loading under hydrogen exposure conditions representative of physiological environments. The results demonstrate pronounced hydrogen localization at the crack tip driven by hydrostatic tension, leading to a strong local reduction in fracture toughness and triggering unstable, cleavage-like crack propagation. Load–displacement responses exhibit abrupt loss of load-carrying capacity, while field visualizations reveal clear spatial correlations between hydrostatic stress, hydrogen coverage, degraded fracture toughness, and crack evolution. The proposed framework provides mechanistic insight into hydrogen-assisted degradation and fracture of biodegradable Mg implants and offers a robust computational tool for assessing implant reliability. The methodology is readily extensible to incorporate plasticity, hydrogen trapping, or corrosion kinetics, enabling predictive modeling of degradation-driven failure in bioresorbable metallic implants.