Wetland ecosystems contain large C stocks and are considered to play an important role in global C cycling, thus having potential implications for global climate change. The Sanjiang Plain wetland is the largest freshwater marsh in China and a principle element of the Wetlands of International Importance with three Ramsar wetland sites (Xingkai Lake, Sanjiang, and Honghe) since 2002. Nevertheless, little is known about organic C storage, and no data combining both long- and short-term C accumulation rates have been reported for this region. We used 10 cores collected from previous investigations to determine radiocarbon age and long-term (apparent) rate of C accumulation (LORCA) based on dry bulk density and organic C content; we used five recent cores representing the three main wetland types in Sanjiang Plain to estimate the recent (apparent) rate of C accumulation (RERCA) inferred from ²¹⁰Pb dating. The LORCA ranged from 5 to 61 g C m^？2 yr^？1 with an average of 22 ± 5 g C m^？2 yr^？1 (±SE), and the RERCA ranged from 170 to 384 g C m^？2 yr^？1 with a mean of 264 ± 45 g C m^？2 yr^？1 (±SE). The average C flux was 0.05 Tg C yr^？1 for herbaceous peatland, 0.02 Tg C yr^？1 for humus marsh, and 0.03 Tg C yr^？1 for marshy meadow and the total soil C pool in Sanjiang Plain wetlands was estimated to be 0.36 Pg C. Our results are in good agreement with other published relevant data and may be useful for predicting global climate change. The Sanjiang Plain wetlands deserve more attention in wetland protection and restoration of the wetland ecosystem and wise use of wetlands for agricultural development.