This study investigated the effect of engine load on the performance and emission characteristics of a compression ignition engine fuelled with watermelon seed oil methyl ester (WOME) modified with metal and oxygenated additives. Watermelon seed oil was extracted and converted to biodiesel through acid esterification followed by alkaline transesterification. The oil yield was 48.7828%, while the biodiesel yield was 85.28 vol.%. Acid esterification reduced the free fatty acid content from 5.4464 to 2.347 mg/g, facilitating effective alkaline transesterification. The physicochemical properties of the produced WOME included a density of 0.881 g/cm³, kinematic viscosity of 3.146 mm²/s, moisture content of 0.0389%, cetane number of 57.34, flash point of 94°C, and cloud point of −6°C, indicating favourable fuel characteristics for compression ignition engine applications. The WOME was blended with petroleum diesel and modified with copper (II) oxide (CuO), copper (II) sulphate (CuSO₄), and acetoacetic ester (Ace), and the resulting fuel blends were tested under varying engine loads. The results showed that engine load significantly influenced fuel consumption, brake specific fuel consumption (BSFC), brake thermal efficiency (BTE), exhaust gas temperature (EGT), and gaseous emissions. At a 30 kg load, the lowest BSFC recorded was 0.2438 kg/kW-hr, while the highest BTE was 38.06%. Increasing engine load generally increased fuel consumption, H₂S, NO₂, CO, and EGT, while CH₄ emissions decreased. The optimized WOME blend demonstrated the best overall engine performance and the lowest overall emissions, with the minimum CH₄ emission of 52.7 ppm. The study concludes that watermelon seed oil is a promising renewable feedstock for biodiesel production and that modification of WOME with CuO, CuSO₄, and acetoacetic ester can improve engine performance and emission characteristics under varying load conditions. The findings demonstrate the potential of additive-modified WOME-diesel blends as a cleaner and more efficient alternative to conventional petroleum diesel in compression ignition engines.