This study investigates the effects of wheat straw-derived biomethanol–diesel blends on the performance and emission characteristics of a diesel engine. Four fuel blends were tested: pure diesel (D100) and biomethanol blends at volumetric ratios of 5%, 10%, and 15% (B5, B10, B15). Experiments were conducted on a single-cylinder diesel engine under four load conditions (25%, 50%, 75%, and 100%). The results show that although the lower heating value of biomethanol increased specific fuel consumption (from 308 g/kWh to 346 g/kWh at full load), its high oxygen content and latent heat of vaporization significantly improved key emission parameters. Under full load, the B15 blend reduced exhaust gas temperature by 29 °C, soot emissions by 25%, and CO emissions by approximately 21% compared to D100. Conversely, nitrogen oxide (NOx) emissions increased from 2165 ppm to 2420 ppm, attributed to the ignition delay characteristics of biomethanol. Overall, blending biomethanol with diesel is an effective strategy for reducing soot and CO emissions; however, further optimization is required to address the trade-off between NOx emissions and fuel consumption.