Interspecific hybridisation increases genetic diversity and generates novel genetic combinations that may increase the resilience of populations to climate change. The rate of hybridisation between coral species can be increased in the lab to generate hybrid stock for reef restoration, the value of which depends on hybrid fitness under relevant environmental conditions. We cross-fertilised coral species and tested hybrid fitness relative to that of their purebred counterparts. In the ocean and under a natural heat wave, F1 coral hybrids performed as well as one of the purebred parental species or had intermediate fitness between the parental species as measured by growth, survivorship, and bleaching resistance. This lack of outbreeding depression in F1 hybrids indicates that interspecific hybridisation could rapidly boost genetic diversity in degraded populations without negative fitness consequences. Moreover, reproductive fitness of F1 hybrids was significantly higher than that of the maternal parent (no comparison with the paternal parent could be made), and F1 interspecific hybrids were able to generate viable F2 hybrid and backcrossed offspring. These findings demonstrate that hybrids between some pairs of species will likely self-propagate following field deployment, facilitating the maintenance and/or restoration of genetic diversity.