The increase in atmospheric CO2 levels can influence the growth of many invasive exotic plant species. However, it is not well-documented, especially for C4 plants, how these growth responses will alter the effectiveness of the world's most widely used herbicide for weed control, glyphosate. We aimed to address this question by carrying out a series of glasshouse experiments to determine if tolerance to glyphosate is increased in four C4 invasive exotic grasses grown under elevated CO2 in nonlimiting water conditions. In addition, traits including specific leaf area, leaf weight ratio, leaf area ratio, root : shoot ratio, total leaf area, and total biomass were measured in order to assess their contribution to glyphosate response under ambient and elevated CO2 levels. Three of the four mature grass species that were treated with the recommended concentration of glyphosate displayed increased tolerance to glyphosate under elevated CO2. This was due to increased biomass production resulting in a dilution effect on the glyphosate within the plant. From this study, we can conclude that as atmospheric CO2 levels increase, application rates of glyphosate might need to be increased to counteract the growth stimulation of invasive exotic plants.