Previous research has shown that elevated CO2 reduces plant resistance against insects and enhances the water use efficiency of C-3 plants, which improves the feeding efficiency of aphids. Although plant mitogen-activated protein kinases (MAPKs) are known to regulate water relations and phytohormone-mediated resistance, little is known about the effect of elevated CO2 on MAPKs and the cascading effects on aphids. By using stably transformed Nicotiana attenuata plants silenced in MPK4, wound-induced protein kinase (WIPK), or salicylic acid-induced protein kinase (SIPK), we determined the functions of MAPKs in plant-aphid interactions and their responses to elevated CO2. The results showed that among all plant genotypes, inverted repeat MPK4 plants had the largest stomatal apertures, the lowest water content, the strongest jasmonic acid (JA)-dependent resistance, and the lowest aphid numbers, suggesting that MPK4 affects plant responses to aphids by regulating stomatal aperture and JA-dependent resistance. Regardless of aphid infestation, elevated CO2 up-regulated MPK4, but not WIPK or SIPK, in wild-type plants. Elevated CO2 increased the number, mean relative growth rate, and feeding efficiency of aphids on all plant genotypes except inverted repeat MPK4. We conclude that MPK4 is a CO2-responsive plant determinant that regulates the molecular interaction between plants and aphids.