Seagrass ecosystems rank among the most productive marine environments on the planet, hosting high cycling rates of a range of globally relevant elemental cycles. While the cycling of reactive oxygen species (ROS) may have important links with other redox-related biogeochemical cycles (i.e. trace metals, carbon, nitrogen, and oxygen), the dynamics of ROS in seagrass ecosystems are poorly characterized. To assess the flux, decay rate, and fate of ROS, this study quantified rates of hydrogen peroxide (H₂O₂) production and decay through in situ incubations of seawater, algae (Polysiphonia sp.), and seagrass (Zostera marina) from a seagrass meadow near Woods Hole, MA, USA. Incubations of Polysiphonia and Zostera indicated average daytime net primary production values ranging from 12.6 to 137.4 µmol g dry wt^-1 h^-1. By comparison, hydrogen peroxide dynamics were much more variable with fluxes ranging from -272 to 150 nmol g dry wt^-1 h^-1, net production rates ranging from -2,212 to 38,000 nmol g dry wt^-1 h^-1 and decay rate constants ranging from 0.314 to 9.09 h^-1. These results suggest seagrass meadows contribute to rapid and dynamic turnover of ROS. In separate incubations, tracing of isotopically labeled hydrogen peroxide (H₂¹⁷O₂) indicated that hydrogen peroxide decay was predominantly regulated by peroxidase-like activity, suggesting that hydrogen peroxide serves primarily as an oxidant of currently unknown reduced substrates. This study documents the rapid cycling and reductive decay pathways of hydrogen peroxide within seagrass ecosystems, laying the foundation for future studies of ROS cycling dynamics in these and other related shallow marine ecosystems.