The brown marmorated stink bug (Halyomorpha halys Stål, 1855) is a globally invasive polyphagous pest that challenges conventional chemical control. We evaluated caffeine-based preparations—alone and combined with chitosan, acetic acid, and ethanol—against adults under laboratory conditions using topical application and 72 h mortality readouts. Among caffeine-in-water treatments, 3% (w/v) yielded the highest mortality (52.5%), indicating an efficacy peak constrained by solubility/precipitation. The most effective overall formulation was 1% caffeine + 1% chitosan + 3% acetic acid, reaching 57.5% mortality and outperforming higher caffeine loads (3–5%). Ethanol as a co-solvent consistently reduced efficacy across concentrations. Patterns across treatments indicate that bioefficacy was driven predominantly by formulation chemistry rather than dose: the chitosan–acetic acid matrix enhanced cuticular deposition, retention, and diffusion of caffeine, whereas high caffeine levels likely triggered detoxification responses and/or reduced bioaccessible dose due to precipitation. By enabling lower active ingredient loads with equal or greater bioactivity, the biodegradable chitosan–acid system improves the environmental profile of caffeine-based insecticides. These results identify a practical, low-complexity path to optimize caffeine delivery for H. halys control and support integration into IPM frameworks. Field validation, testing on earlier life stages, and assessment of non-target effects and resistance biomarkers are warranted to translate these findings into robust, sustainable pest management strategies.