<p>Body mass is one of the important ecological traits for indexing mammal phenotypic quality, determining individual survival and reproductive potential. Assessing the relative role of body mass determinants, such as weather, habitat, or population density, is crucial to predict individual and population responses to the environmental variations. We considered a widespread herbivore of temperate ecosystems, the roe deer <i>Capreolus capreolus.</i> We evaluated body mass responses of different sex/age classes to environmental variables, such as temperature, precipitation (considered only for fawns), habitat composition and heterogeneity (as Shannon index), and population density, throughout 15 years in a central-Italy area. Average temperature and precipitation in the previous spring-summer showed weak effects on fawns, suggesting that milder and wetter conditions during birth and weaning periods may influence the body mass. Shannon index had opposing effects on sexes: it impacted positively female body mass, and negatively male body mass in warmer months. Broadleaved woodlands were associated with higher body mass in fawns compared to agricultural land, probably because the latter is mostly bare during autumn–winter and does not provide abundant food resources. Population density was not a significant explanatory variable of body mass, probably as a consequence of the relatively high environmental quality of the study area. These findings highlight the role of habitat and weather in driving body mass differences and provide insights for individual responses to environmental variation in Mediterranean ecosystems. Results suggest that management actions should consider seasonal and sex-specific ecological needs, particularly to face the climate and habitat changes.</p>