Genetic diversity within coastal foundation species can enhance species and ecosystem resilience to ocean warming and marine heatwaves. However, the effects of diversity on ecosystem function are often context-dependent and mechanisms underpinning, such contingency, remain poorly understood. To test the relationship between genetic diversity and resilience to warming in a coastal foundation species, we planted eelgrass (Zostera marina) pots at two levels of genotypic richness (1 genotype monocultures or 4 genotype mixtures) and exposed these pots to warming events of different frequencies (sustained or alternating) in mesocosms for four months (mid-summer to late fall). Our results revealed that in monocultures warming reduced pot biomass by 15.8% but warming led to overyielding in mixtures by 33.3%. In contrast, mixture biomass at control temperatures underyielded by 13.2%. Overyielding of mixtures during sustained warming was driven by positive complementarity, which appears to be the result of warming-induced shifts in the relative performance of genotypes over time. We propose that high temperature stress created a tradeoff, such that some genotypes experienced greater photoinhibiton during mid-summer while other genotypes were light limited during the late fall. Thus, seasonal differences in temperature and light conditions in the warming treatment generated asynchrony in genotype peak performance, freed genotypes from competitive interactions, and allowed overyielding via complementarity to occur. While we demonstrate that the effects of diversity on ecosystem function depend on environmental context as well as trait variation among genotypes, our results underscore that maintaining or restoring genetic diversity could dramatically improve the resilience of coastal foundation species to future ocean warming.