Seagrasses provide important ecosystem services, including carbon sequestration. However, there are significant gaps in our estimates of seagrass coverage, particularly in the western tropical Pacific. In the present study we assessed the status and extent of seagrass meadows, dominated by Enhalus acoroides, around Guam, the largest and most populated island in the Marianas. The combined above- and belowground biomass of E. acoroides (~2300 g dry weight m–2) is the highest reported for this species and among the highest for all seagrass species. Elemental analysis of C, N and P revealed variations across site and plant part (i.e. above- v. belowground); N : P ratios suggested N limitation. Between 2004 and 2015, seagrass meadows in Guam decreased in total size by 22%, although it is unclear whether this change was part of a long-term trend and whether it was caused by natural or human factors. The high standing stock of E. acoroides suggests that further examination of this species and this region will be needed to better estimate global seagrass carbon stocks.