Thermally-induced bleaching events are driving rapid declines in coral fitness and calcification rates and are expected to increase in frequency and severity in the future. Studying coral capable of surviving in thermally variable, extreme environments can improve our understanding of coral physiological heat tolerance. This study examined how thermal stress affects calcification rates and physiological fitness of Porites lutea originating from a thermally-variable intertidal reef flat in Southeast Phuket, Andaman Sea in an ex-situ heat stress assay. Exposure to 32 °C for 17 days caused a moderate stress response that was manifested mostly in symbiont fitness through significantly lowered chlorophyll a concentrations and visible bleaching reflected by decreasing mean colour scores. The effect on calcification rates was differential and strongly dependent on coral colony. Overall no significant heat effect was found on mean calcification rates due to this high variability in response. Zooxanthellae densities, dark respiration, net photosynthesis and biomass did not change significantly in under thermal stress, while maximum quantum yield and gross photosynthetic rate were affected marginally. Additionally, no mortality of coral nubbins occurred during the experiment. Intertidal P. lutea show a high resilience and acclimation potential to temperature stress which could benefit them in the face of global warming and future bleaching events. However, additional stressors such as high illumination and high-temperature peaks during acute bleaching might amplify negative impacts of thermal stress in natural bleaching events. Further research should investigate whether this high stress tolerance is retained by corals for extended periods of time in the absence of the constant thermal variability of the intertidal. This has important implications when it comes to the colonisation of surrounding reefs by thermotolerant colonies and the possible persistence of thermal resilience in corals over time.