Globally, an estimated 73 ± 6 Pg of carbon is contained in deadwood, representing roughly 30 times the amount of carbon sequestered by forests annually. Decomposition transfers this carbon to the soil, other organisms or the atmosphere, but it is not clear how different biological agents contribute to the decomposition process. Using a novel large-scale termite suppression experiment in old growth tropical forest, we quantify, for the first time, the relative contribution of microbes and termites to deadwood decomposition. Contrary to prevailing understanding, we demonstrate that termites can be responsible for the majority of wood mass loss. Using a dead wood decomposition assay, we found termites were responsible for 58–64% of total mass loss, while microbes carried out 36–42%. Tropical forests are globally important for biodiversity and ecosystem service provision, yet climate change and habitat conversion threaten the functioning of these forests with repercussions for the global biosphere. Our study demonstrates that termite-mediated deadwood decay must be included in global carbon models. These findings will consequently help improve the accuracy of Earth-system models and climate forecasts in the face of global change.