Microplastics (MP) and sea surface temperatures (SST) are predicted to reach levels that can significantly affect marine life by the end of the century. Coral reefs across the world are important and sensitive ecosystems that are already facing other significant pressures, such as climate change. The influence of rising sea surface temperatures caused by climate change is a primary driver of the degrading health of coral reefs through bleaching and habitat death. However, the effects of such interactions on coral reef organisms such as damselfish are still poorly understood. Planktivorous damselfish are abundant inhabitants of tropical coral reefs that significantly contribute to the nutrition recycling of the reef. This study examined the effect of different MP concentrations (0, 1.1 and 11 MP per litre) and SST (ambient, ambient + 1.5ºC and ambient + 3.0ºC) on the aggressive behaviour of spiny Chromis (Acanthochromis polyacanthus). Fish were continuously exposed to polyester (PET) fibres (including control) through an automated system and kept at elevated SST for eight weeks. Fish were recorded during feeding and non-feeding events in order to capture potential aggressive behaviour. Validation of MP uptake showed that fish ingested PET fibres across all MP treatments, with marked increases of PET fibres at higher MP concentrations and elevated SST. MP concentrations and elevated SST affected the observed behaviours separately, with MP concentration influencing the less energy-dependent swoops, and SST affected the more energy dependent chases and the overall aggression experienced by the fish. These results suggest that MP and SST affect damselfish's aggressive behaviour, but without interaction between the two stressors. These findings raise concerns about the effects of higher concentrations of MP and increasing SST predicted for the future.