Sea surface temperatures are projected to rise by 1 to 3°C by the end of the century. On some tropical reefs, summer thermal anomalies presently cause end-of-century temperatures, for days even weeks at a time. Detrimental effects of projected future warming on the behaviour, physiology, and morphology of adult reef fishes have been observed. Here, we tested if the length of exposure to elevated temperature during early development influences the resulting phenotype in the Spiny damselfish, Acanthochromis polyacanthus. Newly hatched offspring from wild-caught pairs were reared for 15 weeks at the average summer sea temperature for the collection location (present-day control) and in four treatments, where they were exposed to a higher temperature (+2°C) for the first 3, 7, or 30 days’ post hatching, or for the entire 15 weeks. We compared body size, burst-swimming responses, critical thermal limits and patterns of sex determination. The results demonstrate that short-term summer thermal anomalies might not have long-term effects on juvenile performance. This has important implications for understanding how the increasing frequency of summer thermal anomalies, such as those recently experienced by the Great Barrier Reef, will affect the ability of reef fish to respond to rising ocean temperatures.