Galls are abnormal plant growths induced by invading organisms which provide resources and protection to gall inducers. Dryocosmus kuriphilus (Hymenoptera: Cynipidae) is an exotic gall wasp that forms galls on the stems and leaves of all chestnut species (Castanea spp.) preventing flowering and normal shoot development, and causing tree mortality. The gall wasp threatens chestnut production and efforts to restore American chestnut to Appalachian forests. Jasmonic acid (JA) is a plant signaling compound that is induced by plant stress and insect herbivory, and is involved in the activation of induced resistance. Exogenous treatment with synthetic JA on a variety of plants activates defenses that can negatively impact a wide range of pests. We treated galls with an exogenous application of JA and a JA inhibitor diethyldithiocarbamic acid (DIECA), and evaluated the effects of treatment on gall wasp galling and mortality, and on the performance of gall wasp associates. JA treatment on chestnut galls and surrounding foliage increased gall size, thickness of the vacuolated parenchyma (nutritive tissues), and tannin content in galls, and decreased parasitism, gall wasp mortality, and pathogen infection. DIECA treatment generally resulted in the opposite effects of JA treatment. This study suggests that induction of JA may be beneficial to the gall wasp by enabling it to produce larger galls which may decrease parasitism, and galls with higher tannin content which may reduce pathogen attack. This is the first study to investigate the effects of JA on a Cynipid gall wasp.