Glycine max (Fabales: Fabaceae) is a key global crop, with over 17 million hectares dedicated to soybean production in Argentina, making it a cornerstone of agricultural systems. To promote sustainable agriculture practices, farmers increasingly use plant-based biostimulants, yet their effects on pest interactions remain unclear. This study evaluates the impact of a polyphenol-based biostimulant applied to soybean seeds on two pests: Spodoptera frugiperda (Lepidoptera: Noctuidae) and Nezara viridula (Hemiptera: Pentatomidae). Yield traits were assessed, while insect bioassays measured the survival and weight gain of larval S. frugiperda and nymphal N. viridula. Additionally, protease inhibitor (PI) levels in leaves and seeds were measured after 24 and 72 hours of herbivory. Biostimulant-treated plants exhibited improved yield parameters, including seed count and plant height. Serine PIs (defensive against lepidopterans) increased in the treated plants’ leaves, but cysteine PIs (defensive against stink bugs) showed no significant differences, even after feeding. The survival and weight gain of S. frugiperda were significantly affected by the treatment, whereas no effect was observed for N. viridula. The results suggest that biostimulant-treated soybeans enhance lepidopteran resistance while having limited effects on stink bugs. These findings highlight the potential of polyphenol-based biostimulants to improve integrated pest management and boost crop productivity in sustainable soybean farming systems.