This paper presents an investigation into the material properties and stress–strain responses of wire arc additively manufactured (WAAM) 5356 aluminium alloy at room temperature and after exposure to elevated temperatures up to 550 °C. A total of fifty-four tensile coupons were extracted from as-built WAAM plates with a nominal thickness of 6 mm, considering three extraction orientations (0°, 45° and 90° to the layer deposition direction). Two cooling methods, including air cooling and water quenching, were adopted following linear heating and isothermal soaking. The geometric features of the coupons, including cross-sectional areas and eccentricities, were captured by laser scanning, while the mechanical properties of the material were obtained through tensile coupon tests. The test results revealed a higher retention of yield and ultimate strengths of WAAM 5356 aluminium alloy after fire exposure than conventionally manufactured aluminium alloys. The intra-layer and inter-layer failure modes of the coupons were found to be associated with their extraction orientations, based on fractographic analysis results. New predictive equations for the post-fire retention factors, together with a constitutive model, for WAAM 5356 aluminium alloy are proposed. The scope of the proposals relating to the ultimate stress and ductility is currently limited to the layer deposition direction, while further experiments are required to extend the proposals to other loading directions.