Additive manufacturing (AM) features repeated thermal cycles due to the track- and layer-wise fabrication process. However, the unique thermal cycling often encourages the precipitation of detrimental phases, such as the isothermal ω phase in metastable β titanium alloys, which cause severe embrittlement. This study aims to address ω phase embrittlement in Ti−13.5Mo (wt%) metastable β titanium alloy fabricated by laser powder bed fusion (L-PBF) through Sn additions. It is shown that 5.0 wt% Sn microparticles can be reliably in-situ alloyed with Ti−13.5Mo by L-PBF to effectively inhibit the formation of the commensurate isothermal ω phase in the binary Ti−13.5Mo alloy. Detailed microstructural characterizations and simulations of the precipitation kinetics reveal that both Ti−13.5Mo with and without Sn exhibit densely populated ω phase throughout the microstructures. However, the Sn addition retards development of the final commensurate form of isothermal ω phase, thereby mitigating its embrittling effects. As a result, Ti−13.5Mo+5Sn fabricated by L-PBF exbibits a good balance of strength and ductility which outperforms those of similar alloys produced by conventional manufacturing routines. Since the Ti−Mo binary system forms the basis of important multicomponent titanium alloys, the finding in this work is expected to be applicable beyond the binary alloy considered here and provides a framework for the design of β titanium alloys for AM that are resistant to ω phase embrittlement.