•European bison fed on 105 plant taxa during growing season in Białowieża Forest.•Woody species (59.4% of DNA sequences) and forbs (33.6%) dominated in bison diet.•The diversity of plants eaten by bison increased with increasing food availability.•Bison continuously adjusts its diet with seasonal availability of non-grass vegetation.•Diet plasticity and use of forest openings allow bison to persist in sheltering forest habitats. Large herbivores that survived the Pleistocene/Holocene transition are hypothesized to have been forced to take refuge, as a result of environmental changes and human pressure, into forest habitats. Today, there is an open question of the degree to which extant large herbivores are well adapted to the forests that allowed for the herbivores’ persistence. We studied the diet of European bison (Bison bonasus), the largest terrestrial mammal in Europe, to gain insight into the foraging behaviour of a large herbivore, that appears to be primarily adapted to grazing but has been restored to forested habitats. The study population resided in the Białowieża Primeval Forests, Poland. DNA-based analysis of faecal samples revealed strong seasonal and spatial patterns in bison foraging, consistent between sexes. Bison fed on at least 105 different plant taxa. Woody species constituted 59.4% of DNA sequences, and forbs 33.6%. The two most abundant taxa were raspberry (Rubus idaeus) and European hornbeam (Carpinus betulus), which together comprised 31.0% of the dietary sequences and occurred in over 80% of faecal samples. Seasonally, the diversity of plants eaten by bison increased with increasing food availability. During high plant biomass in summer, bison consumed up to 40 different plant taxa. There was little overlap in the composition of the diet from month to month, reflecting the strong seasonality of vegetation abundance and/or its dietary quality. The results indicate high plasticity in bison foraging strategies and response to seasonal changes in biomass and the species composition of plants. Bison are browsers which continuously adjusts their diet with seasonal availability of easily digestible non-grass vegetation. We propose that dietary plasticity and micro-selection for open habitats (gaps and river valleys) within a forested landscape allowed bison to persist in sheltering forest habitats during the Holocene and accommodate to forest environments during species restoration.