As one of the most carbon (C) rich ecosystems in the world, the conservation and restoration of tropical wetlands represent a priority for climate change mitigation. Deforestation, agri/aqua culture conversion, pollution and coastal development have all contributed to the loss of over one-third of mangroves in the last fifty years; a significant source of greenhouse gas emissions. This is the context laid out in this study, which seeks to quantify the ecosystem carbon stocks of the Sian Ka’an Biosphere Reserve (SKBR) in the Yucatan Peninsula, Mexico. Such quantification is important in order to establish a baseline that can then be monitored over time and it is a necessary prerequisite for mitigation strategies such as REDD+. The methodology of the study is explained in detail, including a description of the geography, flora and climate of the study site. In total, nine different coastal wetland sites were sampled, representing four kinds of vegetation: tall mangroves (2 sites); medium mangroves (2 sites); dwarf mangroves (3 sites); and marsh (2 sites). Using methodologies outlined by Kauffman and Donato (2012), the authors collected data necessary to calculate the total C stocks from standing tree biomass, downed wood and soil (which was also sampled for Nitrogen (N) and Phosphorus (P) concentration and interstitial salinity). Scaling up the calculations and the statistical analysis used are then discussed. The results showed that tall mangroves had the highest C stocks, followed by the medium and the dwarf mangroves. At all of the sites, soil C comprised the vast majority of the C stocks (between 78-99 per cent), with correlations suggesting that P limits C sequestration and accumulation potential. In total, the coastal wetlands of Sian Ka’an may store 43.2 to 58.0 million Mg of C.