A recognized invasive weed, Canada thistle (Cirsium arvense L. Scop.) was grown at ambient and pre-ambient concentrations of atmospheric carbon dioxide [CO2] (373 and 287 μmol mol−1, respectively) at three levels of supplemental nitrogen (N) (3, 6 and 14.5 mM) from seeding until flowering [77 days after sowing (DAS)]. The primary objective of the study was to determine if N supply limited the potential photosynthetic and growth response of this species to the increase in atmospheric [CO2] which occurred during the 20th century (i.e. approximately 290 to 370 μmol mol−1 CO2). Leaf photosynthesis increased both as a function of growth [CO2] and N supply during the first 46 DAS. Although by 46 DAS photosynthetic acclimation was observed relative to a common measurement CO2 concentration, there was no interaction with N supply. Both [CO2] and N increased biomass, relative growth rates and leaf area whereas root : shoot ratio was increased by CO2 and decreased by increasing N; however, N supply did not effect the relative response to [CO2] for any measured vegetative parameter up to 77 DAS. Due to the relative stimulation of shoot biomass, total above-ground N increased at elevated [CO2] for all levels of supplemental N, but nitrogen use efficiency (NUE) did not differ as a function of [CO2]. Overall, these data suggest that any potential response to increased atmospheric [CO2] in recent decades for this noxious weedy species was probably not limited by nitrogen supply.