Plastic pollution is an increasing threat to the world’s marine habitats and ecosystem processes. The growing impacts of plastic pollution are likely to interact with ocean warming and acidification in coming decades. As plastic often accumulates with macrophyte detritus, the interactions between plastic and ocean climate change have the capacity to alter current and future detrital-based processes, such as nutrient liberation and secondary production. Here, we used manipulative experiments within outdoor mesocosms to examine the impact of multiple stressors (ocean warming, acidification and plastic pollution) on the decomposition of kelp and seagrass detritus. Plastic pollution and ocean temperature were found to significantly affect detrital decomposition and nutrient liberation. Furthermore, we found that the influence of plastic on marine macrophyte decomposition depends on the type of plastic. Overall, our results clearly show that plastic pollution and ocean warming impact marine decomposition, which will have flow on effects to myriad ecosystem processes now and in the future.