This research aims to investigate the impact of various soybean varieties, combined with the application of potassium nitrate (KNO₃), on the growth, yield, bulb characteristics, and antioxidant activity of shallots (Allium cepa) grown in an intercropping system. The study was conducted from May to August 2025 at Tohudan, Colomadu, Karanganyar, Central Java, Indonesia, at an altitude of 105 meters above sea level. The study used a factorial randomised complete block design consisting of three soybean varieties: Biosoy, Grobogan, and Dega, and three KNO₃ concentrations (0, 3, and 6 g L⁻¹). The observed parameters included soybean growth and yield attributes: plant height, number of pods, seed weight, and 100-seed weight. Observations of shallots included growth, bulb yield, colour characteristics, and antioxidant activity (DPPH radical-scavenging capacity, %RSA). The results showed significant effects of variety, KNO₃, and their interaction on the performance of both soybeans and shallots. Biosoy exhibited the highest growth and yield attributes among the soybean varieties tested and also improved shallot bulb productivity in the intercropping system. Intercropping with Biosoy, combined with a moderate KNO₃ application (3 g L⁻¹), enhanced bulb redness (a*) and chroma, indicating greater colour intensity and market quality. The highest antioxidant activity (94.94% RSA) was observed in the Biosoy without KNO3 treatment, whereas higher KNO₃ levels tended to reduce antioxidant potential. These findings suggest that intercropping shallots with the Biosoy soybean variety and moderate KNO₃ fertilization optimizes both agronomic productivity and functional bulb quality, offering a sustainable approach to improving land-use efficiency in limited farmland areas.