Single pass autogenous butt welds, without detect- able volumetric defects, were manufactured by electron beam welding 90-mm-thick UNS S41500—a low carbon 13% Cr- 4% Ni martensitic stainless steel typically used for hydroelec- tric turbine applications. The mechanical properties of the joints were evaluated in the as-welded condition by tensile testing, Charpy V-notch testing, and bend testing. Specifically, static tensile loading was conducted on samples extracted transverse and longitudinal to the weld seam. To determine the impact toughness of the electron beam welded joints, the Charpy tests were conducted on samples with V- notch roots located in the fusion and heat-affected zones at −18 °C. The yield strength, ultimate tensile strength, elongation and toughness of the electron beam welds were found to more than adequately meet the minimum acceptance criteria specified in ASME section VIII and IX. Also, the formability of the welds, examined by bend testing, displayed no discon- tinuities on the tension side of the bent joints. These promising mechanical properties determined for the as-welded UNS S41500 stainless steel offer considerable prospects for indus- trializing the electron beam welding process.