One of the main challenges of present research on plastic pollution is to estimate the amount of plastic debris in the ocean, and understand their origin, fluxes and pathways, and where they ultimately accumulate. The seafloor is a sink for plastic debris. However, little is known on the fate of plastic debris once there. Here we investigate microplastics and larger plastic debris within the so-called “Neptune balls”, formed by debris of the seagrass Posidonia oceanica washed up on beaches of a Mediterranean Sea island. We found up to 1470 plastic items per kg of seagrass, which were essentially filaments and fibers of polyamide and fragments of polyethylene terephthalate, polyethylene, and polypropylene. Our findings show that seagrass meadows ease the accumulation of high-density plastic debris and their entanglement with natural lignocellulosic fibers detached from Posidonia under relatively calm conditions. Plastic trapping Neptune balls are washed ashore during stormy conditions. Our results evidence how a Mediterranean key habitat supporting fisheries, sequestering carbon and providing coastal protection, also counteracts plastic pollution.