Climate change and biodiversity loss are deeply intertwined anthropogenic global crises, for which forests provide powerful nature-based solutions. Biodiverse forests are more resilient to climate change than monocultures, thereby enhancing long-term carbon storage and ecosystem-based adaptation. Awareness of these interdependencies is slowly growing, but we know little about how countries are considering biodiversity within climate policies. Island and low-lying coastal states are particularly vulnerable to climate change and biodiversity loss. Here we assessed if and how the members of the Alliance of Small Island States (AOSIS) are integrating biodiversity into their national climate action plans through forest-based climate solutions. Our analysis shows that these solutions are a missed opportunity for tackling the twin crises together. Only five of the 39 countries explore co-benefits and synergies between forest-based climate solutions and biodiversity conservation measures. An additional nine mention them separately. Among these 14 countries, only a narrow range of interventions were proposed. While 28 AOSIS members prioritised forests for combating climate change, mostly for mitigation, only three prioritised their unique and globally important biodiversity. This omission is potentially risky, since mitigation measures, such as planting rapidly growing non-native trees, can have negative outcomes for biodiversity. Climate action plans must place a greater emphasis on concrete and measurable targets that create synergies with biodiversity conservation, including through the protection of old-growth forests and forest restoration. Our results highlight that forums such as the United Nations Climate Change Conferences need to continue pushing for a stronger integration of biodiversity into climate policies.