Methane emissions from coastal sediments are increasingly influenced by ecological change, including eutrophication-driven seaweed blooms and efforts to restore coastal vegetation. Yet the pathways and variability of methane production in these environments remain poorly constrained. Here, we compared methane production from three seaweeds (Ulva, red filamentous algae, and kelp) and three coastal plants (mangrove leaves, saltmarsh plants, and seagrass) in sands from Port Phillip Bay, Australia. To identify potential predictive precursors, we quantified key methylated osmolytes (dimethyl sulfoniopropionate (DMSP), choline, trimethylamine (TMA), trimethylamine N-oxide (TMAO)), that serve as methanogenic substrates. Methane production from seaweeds was strongly correlated with osmolyte content, whereas coastal plants, particularly mangrove leaves, stimulated methane production despite low osmolyte levels, likely via decomposition pathways generating methanol. These findings broaden the understanding of organic matter sources fueling methanogenesis in coastal sediments and highlight an overlooked contribution of both seaweeds and plants to coastal methane cycling.