Understanding the trophic ecology of the European brown hare (Lepus europaeus) is essential for its conservation in intensifying agricultural landscapes. To explore dietary niches across habitats, sexes, and age groups, we applied stable isotope analysis of carbon (δ 13C) and nitrogen (δ 15N) in hair samples from 151 hares collected in Lithuania and Poland from 2023 to 2025. δ 13C and δ 15N values varied significantly by country and habitat, but not by sex or age. Lithuanian hares exhibited lower δ 13C values and a wider isotopic niche, reflecting more diverse foraging in heterogeneous habitats. In contrast, Polish hares from uniform orchard landscapes showed higher δ 13C enrichment and narrower isotopic ranges, indicating greater dietary specialization on cultivated plants. Temporal variation was minimal, and isotopic overlap among months and years was high. These results suggest that habitat heterogeneity, rather than demographic factors, drives dietary flexibility in L. europaeus. Thus, stable isotope analysis provides a powerful tool for linking agricultural land use with trophic niche breadth in farmland mammals. Promoting mixed cropping systems, permanent vegetation, and structurally diverse habitats could improve food quality and support the recovery of declining hare populations.