Mountains contain many small and fragmented peatlands within watersheds. As they are difficult to monitor, their role in the water and carbon cycle is often disregarded. This study aims to assess the stream organic carbon exports from a montane peatland and characterizes its contribution to the water chemistry in a headstream watershed. High frequency in situ monitoring of turbidity and fDOM were used to quantify respectively particulate organic carbon (POC) and dissolved organic carbon (DOC) exports at the inlet and outlet of a peatland over three years in a French Pyrenean watershed (1,343 m.a.s.l.). The DOC and POC signals are both highly dynamic, characterized by numerous short peaks lasting from a few hours to a few days. Forty‐six percent of the exports occurred during 9% of the time corresponding to the highest flows monitored at the outlet. Despite its small area (3%) within the watershed, the peatland contributes at least 63% of the DOC export at the outlet. The specific DOC flux ranges from 16.1 ± 0.4 to 34.6 ± 1.5 g m2 year−1. POC contributes 17% of the total stream organic carbon exports from the watershed. As the frequency of extreme climatic events is expected to increase in the context of climate change, further studies should be conducted to understand the evolution of underestimated mountainous peatland carbon fluxes and their implication in the carbon cycle of headwaters.