Abstract: Climate change has emerged as a significant challenge threatening the stability and sustainable development of global ecosystems. Enhancing the carbon stock of natural ecosystems, particularly forests, is a crucial strategy in addressing this challenge. As a key component of the United Nations Educational, Scientific and Cultural Organization (UNESCO)′s Man and the Biosphere Programme, Biosphere Reserves (BRs) offer considerable potential in mitigating climate change through forest conservation and sustainable management. However, there is currently a lack of in-depth research that quantifies the effectiveness of BRs in mitigating climate change by assessing the benefits of above-ground forest carbon stock. This study constructs an assessment framework at a global scale for BRs, integrating multi-source data (including ecoregions and biomes, national boundaries, land cover, precipitation, temperature, elevation, slope, distance to cities, population count, and soil organic carbon) and spatial matching (exact matching and similarity matching). A comparative statistical analysis is conducted on the above-ground forest carbon stocks of 617 eligible matched BRs and their external non-BR control, aiming to explore the relative carbon stock benefits of BRs. The findings are as follows. 1) The total carbon stock within the studied BRs (14.106 billion tons) exceeds that of external control regions (13.308 billion tons) by 6.00%. More than half (56.56%) of the BRs have higher internal carbon stocks compared to external regions, indicating a generally positive impact of BRs on forest carbon stock. 2) Although the carbon stock benefits of BRs vary across continents, they all show positive effects, with Oceania demonstrating the highest benefit (+12.52%) and Asia the lowest (+3.20%). 3) Significant differences exist in the relative carbon stock benefits of BRs across different biomes. Tropical and subtropical dry broadleaf forests, flooded grasslands, and savannas exhibit the highest benefits (+13.82% and +13.52%, respectively), while tundra shows the lowest (-5.55%). Temperate conifer forests and montane grasslands and shrublands also display negative benefits (-0.91% and -0.82%, respectively). 4) There are notable differences in the carbon stock benefits of BRs established at different times. BRs established either too early or too recently tend to have relatively lower carbon storage benefits, while those established between 1995 and 1999 demonstrate significantly higher benefits (+40.80%). This study is the first to quantify the positive contribution of BRs to above-ground forest carbon stock on a global scale, and reveal significant differences in carbon stock across various types of BRs. These findings provide a basis for evaluating the role of BRs in mitigating climate change as well as theoretical and data support for optimizing global BRs management policies.