Global climate change, such as elevated atmospheric carbon dioxide (eCO2), may accelerate the breakdown of crop resistance to insect pests by compromising expression of resistance genes. This study investigated how eCO2 (700 μmol/mol) affected the susceptibility of red raspberry (Rubus idaeus) to the European large raspberry aphid (Amphorophora idaei) Börner (Homoptera: Aphididae), using a susceptible cultivar (Malling Jewel) and cultivars containing either the A1 (Glen Lyon) or A10 (Glen Rosa) resistance genes. Compared to plants grown at ambient CO2 (aCO2) (375 μmol/mol), growth rates were significantly increased (ranging from 42–300%) in all cultivars at eCO2. There was some evidence that plants containing the A1 gene were more susceptible to aphids at eCO2, with aphid populations doubling in size compared to the same plants grown at aCO2. Moreover, aphids grew 38% larger (1.36 mg compared with 0.98 mg) on plants with the A1 resistance gene at eCO2 compared with those at aCO2. Aphid performance on plants containing the A1 gene grown at eCO2 was therefore similar to that of aphids reared on entirely susceptible plants under either CO2 treatment. In contrast, aphids did not respond to eCO2 when reared on plants with the A10 resistance gene, suggesting that plants with this resistance gene remained resistant to aphids at eCO2.