Tannins are ubiquitous secondary plant compounds that reduce the biological performance of herbivores by impeding protein metabolism. They may also impact the third trophic level when parasitoids or predators ingest herbivore tissues containing tannins. Microplitis mediator (Haliday) (Hymenoptera: Braconidae) is an important parasitoid of polyphagous moth species such as Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). In this study, we analyzed the direct and indirect effects of tannic acid on M. mediator development and longevity when wasps either fed directly on tannic acid in honey solution, or developed in H. armigera larvae reared on diet containing tannic acid. The longevity of both male and female M. mediator was shortened when they fed on honey solution containing various concentrations of tannic acid. Transgenerational effects were also evident, as fewer F1 offspring cocooned successfully, and those that did so often had extended developmental times and lower mass of cocoon. When parasitoid larvae developed in host larvae fed tannic acid in their diet, the percentage cocooning successfully, mass of cocoon, and cocoon time were all negatively impacted, although developmental times were unaffected. Surprisingly, the longevity of both male and female parasitoids was extended by the highest tannic acid concentration (5 mg/g of host larva). We conclude that both direct and indirect consumption of tannic acid can negatively affect the fitness of M. mediator, and thus breeding for tannin content in cotton to impede herbivore performance may have unintended effects on the third trophic level that may reduce the agronomic benefits.