The extent to which an international mechanism to reduce emissions from deforestation and degradation (REDD+) can also provide biodiversity co-benefits will depend on whether the mechanism results in the retention of forest in countries harboring substantial biodiversity. Countries’ decisions whether or not to participate in REDD+ will be influenced by their national reference level of emissions from deforestation, below which their verified emissions can be credited as reductions. In this article, we explore the reduction in extinctions of forest species achieved under four alternative national reference level designs and three alternative levels of finance for REDD+. We use an 85-country partial equilibrium model and species-area relationships to estimate extinction rates for 2,472 nationally endemic forest-dependent amphibian, bird and mammal species if a REDD+ mechanism had been in place during 2005–2010. Our results indicate that elements of REDD+ that are most effective for climate change mitigation—greater finance combined with reference levels which reduce leakage by promoting broad participation across countries with both high and low historical deforestation rates—also offer the greatest benefits for biodiversity conservation.