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Abstract Trade‐offs among traits are central to life‐history theory and often closely linked to an organism's fitness. Understanding how these trade‐offs vary among populations and across environments is therefore important to more accurately predict species' responses to future climate change. However, the extent to which responses vary across populations remains unknown because few studies investigate intraspecific differences. We performed a full‐factorial split‐brood common garden experiment to test how variation in rearing temperature affects developmental timing and other traits important for survival during diapause in the Glanville fritillary butterfly ( Melitaea cinxia ). Pre‐diapause larvae originating from four regions across a latitudinal cline across Europe were reared at four temperatures (25, 28, 31 and 34°C), and we used a reaction norm approach to test for evidence of genetic differentiation and variation in developmental plasticity across regions. We found clear signs of genetic differentiation in multiple developmental traits, as well as differences in developmental plasticity. Northern larvae entered diapause in the fourth instar when the temperatures were low, whereas southern larvae did so in the fifth instar. As a result, development time is canalized with regards to temperature in northern larvae: due to entering diapause one stage earlier, they develop fast even in the cold, whereas southern larvae always develop slower, especially at low temperatures. As a trade‐off, northern larvae have a lower body mass when reared at cooler temperatures compared to southern larvae, and they show increased plasticity in diapause mass. No clear clinal patterns were found in relative fat content. Our results show that trade‐offs between body size, development time and growth rate can vary within species living across environmental clines, possibly as a consequence of natural selection to local environmental conditions or other genetic constraints. This variation highlights the importance of recognizing the context dependency of relationships between important life‐history traits and their interactions with local environments in predicting species' responses to climate change. Read the free Plain Language Summary for this article on the Journal blog.