This paper reports the first experimental tests on additively manufactured steel plates stiffened by sinusoidal waves, followed by a detailed numerical modelling programme to account for the imperfection sensitivity of these shapes. This innovative study focuses on stiffened plates (plates simply supported along both longitudinal edges), which were experimentally tested employing square hollow section (SHS) stub columns. The experimental study comprised of 5 tensile coupon tests, 15 stub column tests and measurements on geometric accuracy and residual stresses on 316L stainless steel samples made by selective laser melting (SLM). Validating against the experimental results, numerical models accounting for realistic manufactured dimensions and geometric imperfections have been developed. The validated numerical models were used to perform a parametric study considering a wider range of geometries and plate slendernesses. Based on the results of the parametric study, the efficiency of this stiffening method across a range of plate slendernesses has been assessed and optimum wave patterns identified, which were shown to be able to enhance buckling strength up to 50% compared to the flat counterparts. Provisional design equations have also been proposed for SLM 316L stainless steel plates stiffened by the optimum stiffening patterns selected.