To simultaneously enhance the strength−plasticity synergy and resistance to hydrogen embrittlement (HE), the post-annealing treatment was conducted in a laser powder-bed fusion Ti−6Al−4V alloy to introduce reversible transformation. The microstructure, mechanical properties, and HE behavior of the alloy were analyzed by electron back-scattered diffraction, transmission electron microscopy, slow-strain-rate tensile test, hydrogen permeation and thermal desorption spectroscopy. The as-printed sample exhibited high strength but limited elongation and high HE sensitivity. When annealed at 550 °C, the elongation was improved but the hydrogen diffusion rate also increased, thus promoting the formation of brittle hydride. When annealed at 750 °C, the reversible transformation α'→β→α' occurred and an α'/β/α' sandwich structure formed, thereby enhancing HE resistance (reducing the total elongation loss to 12%) while maintaining high strength (~1116 MPa). The introduction of nanoscale β-phase and soft-oriented α' grain significantly inhibited hydride formation and hydrogen-induced crack propagation.