Climate change is pushing temperatures towards intolerable limits for many fishes. It is therefore crucial that we understand the capacity of animals to withstand warming to inform conservation actions. Yet, we currently lack evidence of how intraspecific factors, such as body size or population, affect the capacity of individuals to tolerate warming. Using black bream (Acanthopagrus butcheri) as a focal species, we tested two hypotheses: first, that population-level differences exist in thermal tolerance, with northern populations being more tolerant of warming; and second, that upper thermal limits increase with increasing body size. Black bream grow upwards of 2 kg in size and form genetically distinct populations among estuaries along the West Australian coastline, offering a natural experiment of how intraspecific factors affect temperature tolerance. The first hypothesis was tested by measuring the critical thermal maximum (CTmax) of fish from four distinct populations along an eight-degree latitudinal gradient. The influence of body mass on CTmax was tested across a 6- to 8-fold body mass range. These data provide critical insight into the capacity of black bream to cope with environmental warming. We will highlight the importance of intraspecific factors when assessing a species' vulnerability to climate change.