Changed seasonality and increasing temperatures are hallmarks of the ongoing climate change and challenge the viability of individuals, populations, species, and ecosystems. Recent research suggests that aquatic ectotherms such as fish, especially early life-stages are vulnerable to increasing temperatures. To understand, predict, and possibly mitigate the consequences of climate change, it is thus crucial to investigate the causes of variation in early life-stage thermal performance of ectotherms. Egg size has received considerable attention in this context, but previous studies have been limited to species and population comparisons, neglecting the role of egg size variation structuring variation in thermal performance among individuals within populations. This constrains our mechanistic understanding of egg size as a modulator of thermal performance and how organisms may respond to climate change. To test how intraspecific variation in egg size influences thermal performance, we conducted a split-brood experiment where eggs from 64 female pike (Esox lucius) from the same population were fertilized and incubated at four temperatures until hatching. Results showed that larger eggs had higher hatching performance in low temperatures whereas smaller eggs were more successful in higher temperatures. This suggests that fitness advantages associated with larger eggs may change in the future climate.