Wood-degrading fungi play an important role in the carbon cycle through the deconstruction of lignocellulosic polymers in deadwood (1). The enzymatic and abiotic mechanisms used by white-rot and brown-rot fungi to decay wood have been mostly investigated using cultures made at an ambient dioxygen (O2) concentration (2,3). While such concentration is relevant to the decay at the surface of wood or in biotechnological set-ups with air supply, it does not correspond to the conditions naturally encountered by fungi inside the deadwood, where O2 concentration can be strongly depleted toward anoxia. Moreover, among the requirements for fungal growth, O2 availability is an important parameter for biomass degradation (4). In a field study, we identified the brown-rot fungus Fomitopsis pinicola in the anoxic zone of deadwood. This observation prompted us to investigate its ability to grow and decay wood in vitro. Using different set-ups and methodologies, we demonstrate that F. pinicola is able to grow in the absence of O2 and degrade wood with the secretion of a full set of carbohydrate-active enzymes (CAZymes) targeting cellulose and hemicelluloses. These findings provide a deeper understanding of lignocellulose degradation mechanisms in nature and raise opportunities for the development of bio-inspired anaerobic processes.