This research article explores the potential of applying circular design strategies to marine plastics to improve the health of coastal and ocean ecosystems within the Cascais captaincy. Drawing on evidence from literature reviews, document analyses, two case studies, and thirteen semi-structured interviews with fishermen from Cascais and Ericeira—focused on their working conditions and tools, such as fishing nets, while they performed tasks at the port—the study reveals a promising opportunity for circular design in marine plastics. This opportunity is not only viable for the future but could also yield environmental, social, and economic benefits for the planet, oceans, and nearby communities. To arrive at this conclusion, we followed a structured and scientifically sound methodology. Our findings suggest that further research is necessary, particularly on redesigning fishing gear. Additionally, we developed a diagram and documented the evolution of the Blue Circular Postbranding Project (BCpb) as a potential template for implementing circular design strategies in addressing marine plastics.