Maize–soybean intercropping, a traditional cereal–legume cropping system, has been widely adopted in northern China for its economic and ecological advantages. However, the effects of irrigation and nitrogen (N) application rates on the productivity and resource use efficiencies of this system in the Guanzhong Plain remain insufficiently understood. A two-year field experiment (2023–2024) was conducted to evaluate the effects of irrigation and N fertilization on crop yields, aboveground biomass, and water, N, and radiation use efficiencies. The experimental design included three cropping patterns (maize monoculture, soybean monoculture, and maize–soybean intercropping), two irrigation levels (rainfed and supplemental irrigation of 30 mm), and three N application rates for maize (240, 180, and 120 kg N ha⁻¹). The results showed that supplemental irrigation significantly increased grain yields by 8.46–17.33 % for maize and 19.50–34.91 % for soybean, while enhancing aboveground biomass by 8.85–17.02 % and 11.53–15.97 %, respectively. It also significantly improved crop N uptake, water productivity (WP), nitrogen partial factor productivity (NPFP), and radiation use efficiency (RUE). While a reduction in N application had no adverse effects on maize NPFP or NHI, whereas a 50 % reduction led to significant declines in yield, biomass, WP, and RUE. The intercropping system consistently outperformed monocultures across both years, demonstrating superior resource use efficiency with land equivalent ratios (LER) of 1.08–1.15, ∆WP of 1.15–1.28, ∆NHI of 1.04–1.16, ∆NPFP of 1.14–1.33, and ∆RUE of 1.55–1.64. Based on a comprehensive evaluation of productivity and resource efficiency, the combination of supplemental irrigation (30 mm) with moderate N application (180 kg N ha⁻¹) represents the optimal water–N management strategy for maize–soybean intercropping in the Guanzhong Plain. These findings provide evidence-based water and nitrogen management recommendations for promoting sustainable intensification of cereal–legume intercropping systems in semi-arid regions.