Abstract The design and performance of optimised wire and arc additively manufactured (WAAM) rectangular hollow section (RHS) beams, with varying material strengths and thicknesses along their lengths, are investigated in this study. Simply‐supported beams under three‐point bending were considered. Size optimisation of the RHS beams was carried out by fixing the nominal outer profile and varying the local thicknesses and steel grades in discrete steps to accommodate the variation of the bending moment along the member length. Two optimised beam specimens were manufactured via WAAM and tested, and were shown to have excellent load‐carrying capacities at greatly reduced self‐weights compared with the conventionally manufactured beams, indicating a considerable increase in their structural efficiency. This study confirms the feasibility of combining materials with different thicknesses and steel grades using WAAM, which provides structural engineers with further design freedom and opportunities to minimise material use.