As ocean temperatures rise phenotypic plasticity will play an increasingly important role in coral health. Using ten genotypes of Montipora capitata, we examined if pre-exposure to thermal stress confers improved performance to a second heat stress ~4 months later. Pre-exposure to a constant 31.5 C elicited a strong gene expression response and improved thermal tolerance by up to two degree heating weeks during subsequent thermal testing. The results show improvements from short-term stress hardening in a reef-building coral and substantial capacity for thermal plasticity.