Coral reefs are among the most biodiverse environments on the planet. With the looming threat of irreversible climate change, understanding their tolerance to environmental stressors is key to conserving them. Corals rely on heterotrophy and carbon transfer from autotrophic algal symbionts to survive. Heat stress can interrupt this transfer in a process known as bleaching. Most corals are obligate mutualists, but some are facultatively symbiotic and can live without symbionts. These rely more heavily on heterotrophic carbon, a source shown to facilitate coral survival during bleaching events, especially when their algal symbionts are in low abundance. Here, we test the interactive effects of thermal stress (18-31°C ramp over 3 weeks) and heterotrophy on the facultatively symbiotic coral, Oculina arbuscula. We quantified photosynthetic efficiency of the algae’s photosystem II with Pulse Amplitude Modulated (PAM) fluorometry throughout the 21-day experiment. To understand host response to the treatments, we also quantified total host protein and carb reserves, as well as protein levels of the immune response gene NF-κB. We find that heat stress leads to reduced symbiont density, but reductions are less pronounced in fed corals. We also find that PAM data correlates well with overall symbiont density and that photographic analyses of coral color corroborated these values. Heat stressed and starved corals showed the strongest induction of NF-κB protein levels with a 14-fold increase in protein levels compared to starved controls. Fed and heat stressed corals showed a 6-fold increase in NF-κB levels compared to starved controls. Overall, we find that heterotrophy can mitigate the effects of temperature stress, furthering our understanding of the role heterotrophy plays in coral stress response in facultatively symbiotic species.