How species interactions may modify the effects of environmental change on evolutionary adaptation is poorly understood. Elevated CO2 is known to alter plant–herbivore interactions, but the evolutionary consequences for plant populations have received little attention. We conducted an experiment to determine the effects of elevated CO2 and herbivory by a specialist insect herbivore (Danaus plexippus) on the expression of constitutive and induced plant defense traits in five genotypes of Asclepias syriaca, and assessed the heritability of these traits. We also examined changes in relative fitness among plant genotypes in response to altered CO2 and herbivory. The expression of plant defense traits varied significantly among genotypes. Elevated CO2 increased plant growth and physical defenses (toughness and latex), but decreased investment in chemical defenses (cardenolides). We found no effect of elevated CO2 on plant induction of cardenolides in response to caterpillar herbivory. Elevated CO2 decreased the expression of chemical defenses (cardenolides) to a different extent depending on plant genotype. Differential effects of CO2 on plant defense expression, rather than direct effects on relative fitness, may alter A. syriaca adaptation to changing climate.