Recent work on exotic species in island ecosystems has revealed that many exotic woody plants are capable of dominating forests in which they occur, substantially altering forest structure and nutrient cycling. In mainland forests, however, few empirical examples of exotic dominance exist. The invasive shrub Rhamnus cathartica L. (common buckthorn) is reported to infest temperate forest understories in North America and displace native species, but its degree of dominance has been described only anecdotally. We investigated the extent to which common buckthorn can dominate forest ecosystems, and found strong evidence for monotypic dominance in several mesic and wet sites in southern Wisconsin. Among eight forest sites where common buckthorn was dominant, its mean relative density and basal area was 81% and 45%, respectively. Compared to eight native-dominated sites on similar soils, common buckthorn dominance fundamentally altered forest structure: total woody stem density at Rhamnus-dominated sites was more than twice that of native-dominated sites (two-way ANOVA; P < 0.05, n = 16), but total basal area did not differ significantly (P > 0.3). When considering dominance by size class within only the eight Rhamnus-dominated sites, common buckthorn genets were more abundant than native genets at 5-cm size classes up to and including 20–25 cm diameter at breast height, evidence that common buckthorn dominance can extend well beyond understory size classes. Within Rhamnus-dominated sites, mean relative density and basal area for common buckthorn exceed that reported for four other woody invaders found in the northeastern US, and thus we suggest that common buckthorn is a particularly successful invasive species in eastern temperate deciduous forests of North America and is capable of acting as an ecosystem dominant.