Large herbivores are considered a natural and important determinant for high biodiversity in woodland habitats and have been increasingly reintroduced to nature areas across Europe with the aim of re-establishing natural processes to promote and protect biodiversity. One of the herbivore species playing an increasing role in ecological restoration initiatives is the European bison – the largest extant terrestrial mammal in Europe. However, despite numerous reintroductions of bison for biodiversity purposes, the empirical evidence for their impact is highly limited. Using permanent plots, we investigated the impact of European bison on ground vegetation in forest habitats over a period of eight years. The bison increased the species richness of vascular plants, though mainly to the benefit of graminoids. The effect varied, however, among forest types, with the strongest effect in oak forests with dense and abundant ground vegetation while beech forests characterized by full canopy cover were unaffected. Bison also benefitted bryophytes, which increased in abundance, indicating a generally altered competitive environment with reduced dominance. These results thus provide some of the first empirical evidence, indicating that European bison can promote plant species diversity in forest habitats, by removal of plant biomass, zoochorous seed dispersal, and creation of microhabitat. We thus suggest that introduction of European bison to mixed light woodlands can be a good strategy, benefitting both biodiversity and the conservation of the threatened herbivore species itself.