This study examines the microstructural, mechanical, and electrochemical passivation characteristics of a Ti-35 Nb-5Fe alloy fabricated by selective laser melting (SLM) using elemental powders, with Ti-40 Nb serving as a reference β-type alloy. Fe addition effectively stabilizes the β phase, increasing its fraction from ∼85% in Ti-40 Nb to ∼99% in Ti-35 Nb-5Fe, and promotes substantial grain refinement (from ∼48 µm to ∼11 µm). These microstructural modifications enhance hardness, compressive strength, and plasticity while maintaining a low elastic modulus suitable for biomedical applications. Electrochemical impedance spectroscopy in phosphate-buffered saline at 37 °C reveals the formation of a stable, protective passive film on Ti-35 Nb-5Fe, characterized by higher polarization resistance and lower capacitance during immersion. Mott–Schottky analysis confirms n-type semiconducting behavior with reduced donor density, consistent with Fe³ ⁺-mediated oxygen-vacancy compensation predicted by the point defect model. X-ray photoelectron spectroscopy depth profiling identifies an ∼3 nm thick mixed-oxide layer composed predominantly of TiO₂, Nb₂O₅, and Fe₂O₃. The combined effects of β-phase stabilization, grain refinement, and modified passive-film chemistry demonstrate that SLM-fabricated Ti-35 Nb-5Fe possesses superior passivation stability and mechanical performance, highlighting its potential for load-bearing biomedical implant applications.