Insect parasitoids provide a useful ecosystem service to control pest insects. However, climate change could challenge this pest management, as insects are known to be sensitive to temperature. Furthermore, transgenerational effects, which are common in insects, could influence these effects of temperature on host-parasitoid systems. The present study therefore aimed to test the combined effects of developmental and host parental temperatures on a host-parasitoid system, using the fall armyworm (FAW) and the parasitoid Hyposoter didymator. We focused on mean temperatures of 25 and 29 °C, with a daily fluctuation of ±5 °C. The increase in mean temperatures had a significant effect on all the host parameters tested (survival, developmental rate, sex ratio, body mass) and on parasitoid success. Parasitoid success decreased between the developmental temperatures of 25 and 29 °C, and most effects of the increase in developmental temperature on FAW traits were detrimental to the parasitoid. Remarkably, we found transgenerational effects of temperature on the host resistance to parasitoids (the proportion of FAW escaping parasitism), as well as on host traits associated with the probability of parasitoids finding a host (effects on survival and developmental rates) and host quality (body mass, sex ratio). The parental temperature of 29 °C had a detrimental effect on the FAW resistance to parasitoids, but it reinforced the effects of developmental temperature on host traits that have a negative impact on parasitoids. The study shows the high thermal sensitivity of a host-parasitoid system and highlights that thermal transgenerational plasticity should be considered in host-parasitoid interactions.