Aquatic organisms, including important cultured species, are subject to acute changes in water temperature as the frequency and intensity of extreme weather events worsen. Acute temperature spikes are likely to threaten aquaculture productivity, but dietary intervention may play an important protective role. Therefore, this study used two commercially available diets (20% crude fat versus 10% fat crude fat) to examine if dietary fat content can improve growth performance in juvenile barramundi (Lates calcarifer) while increasing their resilience to acute thermal stress. Fish were fed their assigned diets for 28-days before assessing the upper thermal tolerance (CT_MAX) and the thermal sensitivity of swimming performance and metabolism. We found that feeding fish a high fat diet resulted in heavier fish, but did not affect the thermal sensitivity of swimming performance or metabolism over an 18^oC temperature range (from 20 – 38^oC). Thermal tolerance was compromised in fish fed the high fat diet by 0.48^oC, showing significantly lower CT_MAX. Together, these results suggest that while a high fat diet increases juvenile L. calcarifer growth, it does not benefit physiological performance across a range of relevant water temperatures and may even reduce fish tolerance of extreme water temperatures.