Seasonal variability in grasslands’ vegetation affects animals’ diet selection. We studied the seasonal changes in water buffaloes’ diet during grazing in wet grasslands in Northern Greece. We recorded each month the plant species of the vegetation and the species consumed by buffaloes. We categorized the former into three functional groups (graminoids, legumes, and forbs) and the latter into five groups (graminoids, legumes, forbs, shrubs, and trees). There were significant differences in the proportions (%) of functional groups in the vegetation and in buffaloes’ diet between seasons (χ2 test, P<0.05). Graminoids, legumes, and forbs participated in buffaloes’ diet in all seasons, while the less preferred functional groups were trees and shrubs. Buffaloes consumed ten species in all seasons but we observed the most diverse diet, in terms of plant species, in spring. The most frequently consumed species in each functional group were Cynodon dactylon (graminoids), Trifolium repens (legumes), Cichorium intybus (forbs), Rubus sp. (shrubs), and Populus sp. (trees). However, the majority of plant species in buffaloes’ diet was in very low proportions (<1%), while buffaloes did not sample at all 38 herbaceous species. Researchers need to conduct further research to understand water buffaloes’ foraging strategy regarding plants’ anti-quality characteristics.