A comparison of available data on iron concentration in seagrass tissues shows iron concentrations in seagrass leaves growing on carbonate sediments to be below critical levels for angiosperms [< 100 µg Fe (g DW)− 1]. Iron concentrations of leaves in Caribbean seagrasses declined by 5–10‐fold from terrigenous to carbonate sediments (inner Gulf of Mexico to the Caribbean coast off the Yucatan Peninsula). This observation provided evidence of possible iron deficiency in seagrasses, which was confirmed by an experimental demonstration that adding iron to the sediments stimulates growth of the Caribbean seagrass Thalassia testudinum and increases the chlorophyll a concentration of both T. testudinum and Syringodium filiforme growing above coralline carbonate sediments. We conclude that seagrasses growing above carbonate sediments are likely to experience iron deficiency.