Coastal ecosystems play critical roles in biogeochemical cycling, food webs, climate regulation, and aquaculture. Their functions are shaped by multitrophic interactions across microbe-microbe, microbe-plant, and microbe-animal interfaces, yet our understanding of the underlying mechanisms remains limited. Synthetic ecology offers a promising approach to disentangle such interactions using simplified and controllable synthetic communities (SynComs). Here, we review microbe-plant-animal interactions toward ecosystem function improvements and provide a biological foundation for SynCom design. We further propose a framework for coastal synthetic ecology, including SynCom design and construction, experimental validation of SynCom functions, and laboratory scaling-up and field applications with a focus on greenhouse gas reduction, carbon sequestration, and pollutant degradation. Finally, we discuss future directions for coastal synthetic ecology, with a focus on biogeochemical cycling, food web structure and function, and biological stoichiometry. Overall, this review highlights the potential of SynComs to address environmental and ecological challenges in coastal ecosystems.