Seagrass distribution and sediment biogeochemical conditions were measured around Libong Island in the Andaman Sea, Thailand. Seagrass diversity was moderate for a tropical site (four species found: Cymodocea rotundata, Enhalus acoroides, Halophila ovalis and Thalassia hemprichii), with the diminutive species H. ovalis particularly abundant and perhaps serving as important food for dugongs present in the area. Although the sediment organic matter content was low in the seagrass meadows, the sulfate reduction rates were high (48-138 mmol m^-2 d^-1). A significant positive relationship between belowground biomass and sulfate reduction rates across the seagrass species examined indicates that stimulation of microbial activity in the rhizosphere sediments is controlled by the biomass of roots and rhizomes, rather than by specific seagrass characteristics (e.g., morphology, plant activity). The low iron contents in the seagrass tissues were within the range suggested for nutrient limitation. Sediment sulfur pools, in particular pyrite pools, were high and most of the available iron was bound in sulfides, which may cause iron limitation in seagrass tissues. High sediment sulfide production and sulfide accumulation in concert exert sulfide stress on the seagrasses, as indicated by an accumulation of elemental sulfur in the plants examined, suggesting that tropical seagrasses are prone to sulfide intrusion, similar to findings for temperate seagrasses.