Significance We use a process-based biogeochemistry model to quantify the carbon accumulation for peatland ecosystems in the Pastaza-Marañon foreland basin in the Peruvian Amazon from 12,000 y before present to AD 2100. We find that warming accelerates peat carbon loss, while increasing precipitation slightly enhances peat carbon accumulation at millennial time scales. With these impacts, our simulations suggest that the basin might lose up to 0.4 Pg⋅C by AD 2100, with the largest loss from palm swamp. If this loss rate is true for all Amazonia peatlands, we project that these carbon-dense peatlands may switch from a current carbon sink into a future source in this century.