Previous research has shown that elevated CO₂ reduces plant resistance against insects and enhances the water use efficiency of C₃ 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 attenuataplants 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.