Floating Sargassum is thought to be a sink of blue carbon in that it removes carbon dioxide (CO2) from the atmosphere and sequesters organic carbon by sinking into the deep ocean or being buried in sediments. However, Sargassum that washes into intertidal ecosystems will eventually decompose, releasing greenhouse gases to the surrounding air, groundwater, and surface coastal seawater. In recent years, Sargassum strandings have increased in the Atlantic Ocean, making our understanding of the fate of beached Sargassum critical for determining how much carbon is sequestered by the seaweed. This study investigated greenhouse gas fluxes of decaying Sargassum wrack in mesocosm experiments to understand how beached Sargassum could affect the coastal carbon cycle. GHG fluxes were investigated in two mesocosm experiments, simulating (1) tidal seawater inundation of Sargassum wrack and (2) wrack decomposing in oxygen restricted and aerated conditions. Measurements of greenhouse gas fluxes from naturally decomposing Sargassum were also undertaken. Overall, strong CO2 fluxes were recorded with weak or inconsistent CH4 fluxes. Simulated tidal conditions and simulated hypoxic conditions did not lead to differences in GHG fluxes, though dry conditions did have greater CO2 fluxes than seawater immersion conditions. Over the course of the experiment, alkalinity in seawater mesocosms increased, likely due to anaerobic chemical pathways. In-situ incubations of stranded Sargassum returned strong CO2 and CH4 fluxes, although it is unclear whether these were due to the wrack itself or underlying sediment biogeochemical processes.