The vast undisturbed mires in western Siberia formed a significant sink of atmospheric carbon dioxide during the Holocene. However, the lack of spatially resolved simulation models hampers the quantification of Holocene carbon accumulation of the entire mire systems. Here we developed a three-dimensional dynamic model, based on a hydrological approach. We applied the model to a large mire complex in western Siberia to simulate its Holocene development and to quantify the long-term carbon accumulation rate (LORCA). Our model simulated a LORCA with a spatial variation of 10--85 g C m-2 yr-1. The average over the Holocene was 16.2 g C m-2 yr-1. Simulation scenarios for the 21st century show that the average LORCA dropped to 5.2 g C m-2 yr-1 because of oxidation and decomposition of peat following mire drainage after the 1950s. Even in the undrained parts of the mire complex the LORCA is lowered substantially because of the dropped water table and resulting decrease in peat growth. Our results show that carbon accumulation in western Siberian watershed mires will continue in the future. Thus these mires might remain a significant sink of carbon.