Abstract <p>Recurring influxes of pelagic <i>Sargassum</i> have affected the Caribbean since 2011, generating significant environmental, economic, and public health impacts. At the same time, this unprecedented biomass accumulation represents a potential feedstock for sustainable agricultural inputs within circular bioeconomy frameworks. This review synthesizes scientific literature, regional reports, and patent evidence (2015–2025) on the valorization of pelagic <i>Sargassum</i> for compost, biofertilizers, and biostimulants across the Caribbean basin. Particular attention is given to the variability in elemental and biochemical composition among morphotypes and regions, including macro- and micronutrients, organic compounds (alginates, phenolics, and proteins), and priority contaminants such as arsenic and cadmium. Recent basin-scale studies demonstrate that both total arsenic and its inorganic fraction are strongly influenced by nutrient regimes and oceanic transport pathways, with direct implications for risk assessment and regulatory compliance. Heavy metal concentrations frequently approach or exceed agricultural guideline thresholds, underscoring the need for pretreatment (e.g., desalination and washing), arsenic speciation analysis, and batch-specific quality control prior to agronomic use. Despite this compositional heterogeneity, pelagic <i>Sargassum</i> exhibits agronomically relevant nutrient profiles and bioactive compounds that support its development as a soil amendment or biostimulant, provided that safety protocols are rigorously implemented. The review identifies key knowledge gaps, including morphotype-specific baselines, harmonized analytical standards across Caribbean countries, and improved regulatory alignment. Transforming <i>Sargassum</i> from a coastal nuisance into a controlled bioresource requires integrated monitoring, risk-based management, and regionally coordinated policies linking oceanographic dynamics with agricultural valorization strategies.</p> Graphical abstract <p></p>