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Fish size at maturation influences lifetime reproductive success and is an important parameter in managing stocks. Fish tend to reach maturity at a smaller size in warmer water; however, the generality of this pattern is a matter of controversy. The mechanisms by which temperature influences fish size at maturation are not well understood, particularly in natural populations, but may have broad implications if climate change continues to warm the seas. In this study, we use populations of 16 fish species across the Mediterranean Sea to evaluate the association between different temperature metrics and fish size at maturation, and to understand the variation among species. We found that both mean annual temperature and growing degree days (GDD) were the best supported environmental predictors of fish size at maturation. This suggests that the mechanisms affecting size at maturation may differ from those affecting maximum size, for which maximum temperature was the best predictor. Across species, we found that the effect of temperature is stronger for more active species, while other species-level predictors had limited influence. The correlation of fish size at maturation to specific temperature metrics should help fisheries and conservation programmes better predict the effects of climate change on fish populations.