Plastic waste negatively impacts ecosystems, public health, and local economies in Tanzania. For example, marine plastic and microplastic wastes contaminate beaches, sea grass areas and coral reef habitats, lower the quality of marine ecosystems and biodiversity, and endanger bird and marine wildlife through entanglement and the ingestion of plastics of different sizes. They also endanger human health through food chains. Valuation of the costs of environmental degradation (COED) from marine plastic pollution helps the country to understand the scale of the impacts and prioritize activities for mitigation of these impacts. This study developed the methodology to estimate the COED in the selected coastal areas in Zanzibar and Dar es Salaam. It is the first of its kind for valuing the impacts of marine plastic wastes on local economic sectors, public health, and marine ecosystems. The valuation results reveal that marine plastics pollution causes a net economic cost to the local economic activities, especially tourism, and the natural environment of study areas, and that in some areas costs can be quite significant. The analysis of the costs across study areas and sectors is useful for prioritizing marine plastic pollution management activities.