The majority of wastewater produced on land by anthropogenic activities is discharged, with varying levels of treatment, into the aquatic environment. The contaminants contained in this wastewater, in particular nitrogen, phosphorus, metals, and hydrocarbons, accumulate in coastal waters, causing pollution and disturbances in the balance of marine ecosystems. Bioremediation is a sustainable treatment method that reduces the impact of this pollution on the marine environment by using biological organisms, with an increasing focus on seaweeds, to neutralize these contaminants. Recent research and commercial developments in seaweed cultivation demonstrate the potential of seaweeds to remediate nutrients and metals from land-based and coastal aquaculture, urban and agricultural runoff, and industrial facilities. In this chapter, we provide examples of the cultivation of Ulva to treat nutrients from land-based aquaculture, the cultivation of kelp to remediate anthropogenic nutrients in the nearshore coastal environment, and finally seaweed biomass to sequester dissolved metals in industrial effluents. These examples show that despite the technical challenges in implementing this technology at scale, seaweed-based bioremediation provides an opportunity to effectively clean polluted seawater in an ecologically friendly manner. Additionally, these examples demonstrate the significant advantage of producing a valuable commercial crop for use in a growing industry based on seaweed products.