Magnesium (Mg) has gained a lot of attention for its biodegradable properties. Additive manufacturing of Mg would further provide an opportunity to manufacture optimized structures for implants. However, much work remains to ensure stable processes and to understand the correlation between processing parameters, microstructure and related properties. The aim of this study was to process gas-atomized WE43 powder by Powder Bed Fusion-Laser Beam (PBF-LB) in a commercially available machine (EOS M290). The built material was characterized in terms of microstructure and composition and compared with the virgin powder. Thermodynamic computations based on the Calphad-method were for the first time employed to an additively manufactured Mg-alloy to predict phases present. Samples with above 99% density were successfully manufactured. The phases predicted using computational thermodynamics partly overlapped with phases observed experimentally. The study confirms the feasibility of using commercial PBF-LB systems for processing Mg-alloys, and the applicability of thermodynamic modelling to this system.