The populations of the endangered mountain gazelle (Gazella gazella), which inhabit large parts of Israel, across various ecosystems and climatic conditions, shrunk drastically over the last decades. To date, data on gazelle nutrition, how these relate with individual characteristics and respond to seasonal and environmental changes, have not been available. We analyzed 110 samples from gazelle rumen contents collected throughout the country from occasional fatalities, mainly roadkills, and tested the feasibility of using them for near-infrared spectrometry (NIRS) calibrations. Although NIR calibrations for crude protein, in vitro dry matter digestibility, and ash were reasonable, we found that using calibrations based on local forage and feed plant species performed better, and used these to estimate several nutritional constituents in gazelle rumens, using NIRS. We tested how constituents relate to the sex, age-class, and weight of the individual gazelle, and to season and ecosystem type, and found that season plays a major role in gazelle nutrition. Winter is the most propitious season, when crude protein, ash and digestibility are highest, and acid detergent fiber (ADF), neutral detergent fiber (NDF), and the carbon to nitrogen (C:N) ratio are lowest. Autumn, being the harshest season, mirrors winter conditions, and summer and spring show intermediate levels. Yet the relative changes between seasons were mild: about 30%, for crude protein, digestibility, and ash, and 14-22% for ADF, NDF, and C:N ratio. Ecosystem type affected several constituents, and nutrition was slightly better in Mediterranean than in dry ecosystems. Gazelle sex, weight, and age-class had minor effects on nutrition. Overall, it seems that the adaptation of gazelles to their environment is germane to keeping relatively steady nutrition throughout the year. Our results, which do not show a dramatic decline in the quality of gazelle nutrition during any season or among the climatic regions that were studied, suggest that nutrition is not a major driver of the survival of gazelles in the populations surveyed.