not-yet-known not-yet-known not-yet-known unknown Carbonyl sulfide (COS), an atmospheric gas used as a tracer in carbon cycle studies, has an inferred missing sink in high Northern latitudes. Boreal COS budgets typically account for the contribution by forests and ignore any uptake that wetland ecosystems, widespread in Northern latitudes, may contribute. The first direct measurements of the ecosystem-atmosphere COS exchange of a boreal wetland, presented here, demonstrate their likely importance in that Northern latitude COS budgets. The investigated wetland (Siikaneva, Finland) took up on average 11 pmol m−2s−1 COS, which was ~72 % of the nearby boreal forest COS uptake. During nighttime, the COS uptake rates were similar at both sites. Upscaling our measurements to the boreal region using the ORCHIDEE model revealed in a Northern wetland sink of ~13 Gg S/y, changing the budget model output from a small source to a COS sink impacting Northern latitudes carbon uptake estimates based on COS.