Amazonian dark earth (ADE) is highly nutrient- and carbon-rich soil created by past inhabitants of the Amazon. It would be valuable to know the extent of ADE because of its cultural and environmental importance, but systematic efforts to map its distribution and extent are impractical with traditional field methods. We use remote-sensing imagery and a machine-learning classifier with ground-truthed training data to predict the occurrence of ADE across the 26,000 km2 Território Indígena do Xingu (TIX) in the southeastern Amazon region of Brazil. We find widespread ADE across the TIX, well beyond previously studied archaeological sites, occupying at least 3–4% of the land area. We further estimate that the TIX may sequester 9 Mt of carbon within ADE deposits from past human inputs. Our findings show that ancient inhabitants of the TIX substantially modified their environment, highlighting the importance of conserving this natural and cultural resource given threats from climate change and deforestation.