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Insect thermal tolerance affects survival and distribution of species but can vary within and between individuals due to the environmental conditions they experience. The fall armyworm (FAW), Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), recently invaded Africa and its local hotspots are associated with weather and crop phenology. We investigated the effects of larval diet (maize plants, wheat plants or chickpea‐based artificial diet), life stages and acclimation temperature on thermal tolerance traits of FAW to explore how these variables might contribute to its presence in the field. First and sixth instar larvae and adults reared on each diet at 25°C were acclimated for 24 h at 20, 25 or 30°C. We then recorded the critical thermal minimum (CTmin) and critical thermal maximum (CTmax) of individuals. Sixth instars had the highest CTmin but the effects of acclimation and diet on this trait depended on the life stage being tested. CTmin of first instars increased with acclimation temperature when fed on an artificial diet, but sixth instars and adults were not affected by acclimation or larval diet. CTmax was lowest among adults but acclimation and diet again had effects that differed between life stages. CTmax of first instars and adults increased with acclimation temperature but not in sixth instars. Sixth instars and adults reared on the artificial diet had the highest CTmax but diet had no effect on first instar CTmax. Our results show the complexity of thermal tolerance across FAW life stages and suggest the need to consider local temperature variation and available dietary resources when predicting their potential distribution. [ABSTRACT FROM AUTHOR]