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Coastal ecosystems produce and sequester significant amounts of carbon (“blue carbon”), which has been well documented in humid and semi‐humid regions of temperate and tropical climates but less so in arid regions where mangroves, marshes, and seagrasses exist near the limit of their tolerance for extreme temperature and salinity. To better understand these unique systems, we measured whole‐ecosystem carbon stocks in 58 sites across the United Arab Emirates (UAE) in natural and planted mangroves, salt marshes, seagrass beds, microbial mats, and coastal sabkha (inter‐ and supratidal unvegetated salt flats). Natural mangroves held significantly more carbon in above‐ and belowground biomass than other vegetated ecosystems. Planted mangrove carbon stocks increased with age, but there were large differences for sites of similar age. Soil carbon varied widely across sites (2–367 Mg C/ha), with …