Wire arc additive manufacturing (WAAM) is a metal 3D printing technique through which parts are built up in a layer-upon-layer fashion using metal wire and a welding arc. This paper presents an experimental programme to investigate the residual mechanical properties of WAAM ER70S-6 steel after exposure to elevated temperatures. A total of thirty-six tensile coupons were extracted from as-built WAAM plates fabricated using ER70S-6 feedstock wire, with two nominal thicknesses of 3 mm and 8 mm, two coupon extraction orientations (i.e. parallel and perpendicular to the layer deposition direction) and eight exposure temperatures ranging from 300 °C to 1000 °C. The geometric features of the WAAM steel coupons were captured via 3D scanning, while their stress–strain curves and residual material properties after exposure to elevated temperatures were obtained through tensile testing. Scanning Electron Microscopy (SEM) was used to investigate the fracture surfaces of the tested coupons with different plate thicknesses and exposure temperatures. On the basis of the experimental results, a new set of retention factor curves was proposed to predict the post-fire material properties of WAAM ER70S-6 steel and shown to result in accurate predictions of Young’s modulus, yield stress, ultimate stress, ultimate strain and fracture strain.