<p>Habitat fragmentation poses a significant threat to primate populations, necessitating behavioral adaptations in activity patterns and feeding strategies. This study investigated the impact of habitat fragmentation on the endangered San Martin titi monkey (<i>Plecturocebus oenanthe</i>) in the Moyobamba region of Peru. Activity budgets and feeding strategies were compared between groups inhabiting large (400 ha) and small (5 ha) forest fragments. Results revealed that resting was the predominant activity in both habitats, followed by feeding. Notably, individuals in large fragments allocated significantly more time to feeding (30.1%) than those in small fragments (23.0%) did. Social interactions were more prevalent in the small fragments (15.1%) than in the large fragments (9.1%). Fruit was the primary dietary component in both sites, accounting for 59.9% in the large fragment and 44.9% in the small fragment. However, individuals in small fragments exhibited a higher consumption of leaves (13.7%) and prey (14.6%) than those in large fragments (7.1% and 8.5%, respectively). The feeding time for specific food resources varied according to the fragment size and seasonal availability. These findings demonstrate the ecological plasticity of <i>P. oenanthe</i> in adapting to fragmented habitats by modifying their activity budgets and feeding strategies. However, continued habitat degradation may compromise these adaptive mechanisms, highlighting the need for conservation efforts focused on habitat restoration and connectivity to ensure long-term persistence of this endangered primate species.</p>