Nonhuman primate populations are facing widespread conversion of their habitat to human-modified landscapes dominated by agriculture, in which cultivated species may constitute alternative food resources, particularly during periods of wild food scarcity. We assessed intraseasonal variation in dietary composition and diversity of the western chimpanzee (Pan troglodytes verus) at Lagoas de Cufada Natural Park, Guinea-Bissau, a forest–savannah mosaic disturbed by humans, in relation to food availability. We also investigated spatial variation in dietary composition. We collected phenological data from March 2011 to February 2012 by sampling focal plant taxa and conducted macroscopic analyses of fecal samples and feeding remains during the dry season (February–May and October–December) of 2011. More fruits were available in the dry than in the wet season, and ripe fruit availability peaked in the late dry season. Chimpanzees showed a fruit-based diet composed of 31 identified plant species. Fecal samples (N = 210) were dominated by wild species (82 % volume), while cultivated species were rare (0.9 % volume; 17 % volume of unidentified species). The consumption of fruit species increased with ripe fruit availability, but a few wild fruit species were selected disproportionately to their overall availability. There was no association between dietary composition and distances among fecal samples, suggesting that chimpanzees have access to and largely use the same set of plant species over the entire study area. Moreover, the proximity to agricultural areas did not influence dietary composition. Our findings highlight that chimpanzees in this human-modified landscape still rely mostly on the consumption of wild fruit species and rarely include cultivated foods in their diet. Overall, our study underscores the importance of knowledge of feeding ecology to understand better the effects of anthropogenic habitat modification on primate diet and distribution as well as the limits to their persistence in the expanding human-dominated agricultural landscapes across their range.