<p>The peatlands in Kalimantan exhibit diverse geomorphological characteristics, but their initiation timing and drivers remain unclear due to limited chronological data. Using 55 radiocarbon ages and Bayesian age‒depth modeling of 15 peat cores, we reconstructed the development and carbon accumulation histories of inland and coastal peatlands in West and East Kalimantan. Coastal peat initiation occurred during the middle Holocene, coinciding with postglacial sea-level high stands, whereas inland peat formation began in the late Pleistocene. Carbon accumulation rates peaked in the middle Holocene (coastal: 63–72&#xa0;g C m⁻²&#xa0;yr⁻¹; inland: 53–89&#xa0;g C m⁻²&#xa0;yr⁻¹) under stable hydrological conditions but declined in the late Holocene (coastal: 49–55&#xa0;g C m⁻²&#xa0;yr⁻¹; inland: 58–63&#xa0;g C m⁻²&#xa0;yr⁻¹). The total decline rate was insignificant, at approximately 0.68 Mt C yr⁻¹. These findings indicate that a sustained water balance is crucial for long-term peat growth and carbon sequestration and that hydrological disruption reduces the carbon storage. The current decline in the carbon sequestration capacity of drained tropical peatlands in Kalimantan is 32.4 Mt C yr⁻¹ (118 Mt CO₂-eq yr⁻¹) over a 40-year period, representing approximately 47.5 times the natural decline in carbon sequestration over the past 4000&#xa0;years.</p>