Elevated CO (eCO) and high temperatures are known to affect plant nitrogen (N) metabolism. Though the combined effects of eCO and chronic warming on plant N relations have been studied in some detail, a comprehensive statistical review on this topic is lacking. This meta-analysis examined the effects of eCO plus warming on shoot and root %N, tissue protein concentration (root, shoot and grain) and N-uptake rate. In the analyses, the eCO treatment was categorized into two classes (<300 or ≥300 ppm above ambient or control), the temperature treatment was categorized into three classes (<1.5, 1.5-5 and >5 °C above ambient or control), plant species were categorized based on growth form and functional group and CO treatment technique was also investigated. Elevated CO alone or in combination with warming reduced shoot %N (more so at ≥300 vs. <300 ppm above ambient CO), while root %N was significantly reduced only by eCO; warming alone often increased shoot %N, but mostly did not affect root %N. Decreased shoot %N with eCO alone or eCO plus warming was greater for woody and non-woody dicots than for grasses, and for legumes than non-legumes. Though root N-uptake rate was unaffected by eCO, eCO plus warming decreased N-uptake rate, while warming alone increased it. Similar to %N, protein concentration decreased with eCO in shoots and grain (but not roots), increased with warming in grain and decreased with eCO and warming in grain. In summary, any benefits of warming to plant N status and root N-uptake rate will generally be offset by negative effects of eCO. Hence, concomitant increases in CO and temperature are likely to negate or decrease the nutritional quality of plant tissue consumed as food by decreasing shoot %N and shoot and/or grain protein concentration, caused, at least in part, by decreased root N-uptake rate.