The carbon (C) dynamics of tropical peatlands can be of global importance, because, particularly in Southeast Asia, they are the source of considerable amounts of C released to the atmosphere as a result of land-use change and fire. In contrast, the existence of tropical peatlands in Amazonia has been documented only recently. According to a recent study, the 120 000 km2 subsiding Pastaza-Marañón foreland basin in Peruvian Amazonia harbours previously unstudied and up to 7.5 m thick peat deposits. We studied the role of these peat deposits as a C reserve and sink by measuring peat depth, radiocarbon age and peat and C accumulation rates at 5–13 sites. The basal ages varied from 1975 to 8870 cal yr bp, peat accumulation rates from 0.46 to 9.31 mm yr−1 and C accumulation rates from 28 to 108 g m−2 yr−1. The total peatland area and current peat C stock within the area of two studied satellite images were 21 929 km2 and 3.116 Gt (with a range of 0.837–9.461 Gt). The C stock is 32% (with a range of 8.7–98%) of the best estimate of the South American tropical peatland C stock and 3.5% (with a range of 0.9–10.7%) of the best estimate of the global tropical peatland C stock. The whole Pastaza-Marañón basin probably supports about twice this peatland area and peat C stock. In addition to their contemporary geographical extent, these peatlands probably also have a large historical (vertical) extension because of their location in a foreland basin characterized by extensive river sedimentation, peat burial and subsidence for most of the Quaternary period. Burial of peat layers in deposits of up to 1 km thick Quaternary river sediments removes C from the short-term C cycle between the biosphere and atmosphere, generating a long-term C sink.