<p>Coastal wetlands are important land–ocean interfaces for organic carbon storage. It was assumed that reactive poorly crystalline or short-range-ordered iron minerals enhance organic carbon accrual and persistence. However, these metastable minerals are prone to rapid reductive dissolution, raising uncertainties about their abundance and impact on carbon cycling in coastal wetlands, where anoxia typically prevails. Here we combine a global database of ~23,000 observations and a national survey across China’s coastline. We show that coastal wetlands are enriched preferentially in poorly crystalline or short-range-ordered iron minerals over well-crystalline phases, compared with uplands. Mössbauer spectroscopy reveals that ferrihydrite, nanogoethite and highly disordered phases dominated the iron oxide pool in coastal wetlands, with minor crystalline forms, challenging the notion that reactive metastable minerals are removed preferentially under anoxic conditions. We find that reactive metastable iron was most abundant in tropical wetlands, in contrast with tropical uplands where crystalline minerals predominate. Despite a higher abundance of metastable iron minerals, coastal wetlands had a similar fraction of total organic carbon associated with iron oxides (~13%) to uplands. Furthermore, these widespread metastable iron minerals show no evidence of organic carbon saturation in coastal wetlands, highlighting potential for boosting this rusty carbon sink through management practices.</p>