As atmospheric carbon dioxide concentration ([CO2]) increases, it is anticipated that the competitive ability of C3 crops could be enhanced relative to C4 weeds in agricultural systems. However, given the different nitrogen use efficiencies of C3 and C4 plants, it is unclear whether any effect of increasing [CO2] on C3/C4 competition is nitrogen dependent. To determine the interaction of [CO2] and N availability on species growth and competitive outcomes, the growth of rice (C3 photosynthetic pathway) was examined in both monoculture and in competition with a common weed, barnyardgrass (C4 photosynthetic pathway) at two levels of N supply (0.357 and 1.071 mmol N L−1) and two levels of [CO2] (ambient and ambient + 200 µmol mol−1) under field conditions in eastern China. In monoculture, the biomass response of rice to elevated [CO2] depended on N supply, whereas the response of barnyardgrass to elevated [CO2] was less dependent on nitrogen. Consequently, when grown in mixture, the proportion of rice biomass increased relative to that of barnyardgrass under elevated [CO2] if the supply of nitrogen was adequate. However, if N was low, elevated [CO2] significantly reduced the proportion of leaf area and root biomass relative to barnyardgrass biomass. Although data from this experiment confirm that competitiveness of rice could be enhanced relative to C4 weeds in response to rising [CO2] in situ, the data also indicate that such a response could be contingent on the supply of nitrogen. This suggests that, for rice cropping systems where N is in limited supply, rising atmospheric CO2 could still exacerbate competitive losses, even from C4 weeds.