Peanut (Arachis hypogaea) is an important legume crop and a vital source of plant based protein. However, its cultivation faces challenges from biotic stresses, including competition with allelopathic weeds like Amaranthus spinosus. Interestingly, allelochemicals from this weed can exhibit a hormetic effect, acting as biostimulants at specific concentrations. This study aimed to examine the allelopathic potential of various concentrations of Amaranthus spinosus extract on the physiological activity of two peanut varieties. The research was conducted from July to September 2025 at the Faculty of Agriculture Greenhouse, Universitas Sebelas Maret Surakarta, using a factorial randomized complete block design (RCBD). The first factor was peanut variety (Talam 2 and Hypoma 2), while the second factor was extract concentration (0%, 0.5%, 1%, 1.5%, and 2%). The results showed a significant interaction between variety and extract concentration on chlorophyll b and total chlorophyll content. The application of 0.5% extract concentration increased chlorophyll a content (0.6387 mg g-1), stomatal aperture width (8.18 µm), and stomatal aperture length (17.36 µm). Meanwhile, 1% extract concentration increased stomatal conductance (0.2872 µmol m-2 s-1) and transpiration rate (0.0290 µmol m-2 s-1). Talam 2 variety showed the highest leaf greenness (47.29 SPAD). These findings demonstrate that the allelopathic effect of A. spinosus extract exhibits a biphasic hormetic response, where low concentrations function as biostimulants while higher concentrations induce physiological stress, with variety specific responses in peanut plants.