The increasing global population, projected to reach nearly 10 billion by 2050, poses significant challenges to resource sustainability, particularly in terms of food, water, and energy supply. A transition towards a bioeconomyBioeconomy, utilizing renewable biological resources instead of fossil fuels, is essential for a sustainable future. Marine macroalgae, commonly known as seaweed, have emerged as a promising alternative biomass source due to their rapid growth, high biomass yield, and diverse applications in biofuel production, pharmaceuticals, food ingredients, and environmental remediation. This chapter explores the potential of macroalgae biorefineries in the framework of the “Blue Economy,” which envisions ocean-based economic growth while ensuring environmental sustainability. The classification of macroalgae into red, brown, and green species highlights their unique compositions and industrial applications. The focus is placed onUlva cultivation Ulva sp., a green macroalga with high economic value cultivated in the Eastern Mediterranean Sea. To achieve large-scale sustainability, offshore seaweed cultivation is proposed as an alternative to onshore farming, minimizing land competition and environmental impact. However, technological challenges such as nutrient limitations, cultivation structures, and biomass processing must be addressed. This chapter also introduces artificial upwellingArtificial upwelling as a potential method for nutrient enhancement in offshore environments, paving the way for a viable marine biomass-based bioeconomyBioeconomy.