Mangroves are increasingly regarded as a cost-effective nature-based climate solution for critical ecosystem services they provide, including habitat for terrestrial and aquatic fauna, biodiversity conservation, and protection from cyclones. It has repeatedly been claimed in the literature that mangroves store greater amounts of carbon (C) than any other terrestrial ecosystems. This claim, however, is not supported by enough empirical data, particularly from South Asia. This study provides a comprehensive scenario of the C storage and sequestration potential of mangroves along a salinity and successional gradient in the Sundarbans mangrove forest – the world’s largest contagious mangrove forest shared between Bangladesh and India. Carbon stocks and fluxes were monitored along three successional (pioneer, secondary, and climax forest) and salinity gradients (i.e. oligohaline, mesohaline, and polyhalite). The authors used a stratified random sampling approach, and data were collected both through destructive and non-destructive sampling. Carbon stocks were measured in tree biomass, undergrowth, litter/woody debris, fine root, and soil. Both autotrophic and heterotrophic respiration and litterfall were measured as an indicator of C flux in the forest. This study suggests the highest C stock in the climax forest and less saline zone dominated by Sundri (Heritiera fomes), while the largest C sequestration potential is in the pioneer forest and newly accreted land dominated by Keora (Sonneratia apitala). This study would be useful for scientists and policymakers to recognize the potential of Sundrabans forest as a nature-based climate solution.