Fucoxanthin (Fx), a marine xanthophyll carotenoid, has attracted considerable scientific attention due to its wide-ranging biological activities, including antioxidant, anti-inflammatory, anti-obesity, and anticancer effects. Despite its substantial therapeutic potential, the clinical application of Fx and its derivatives remains constrained by their structural complexity, low chemical stability, and limited bioavailability. This review offers a thorough and up-to-date overview of Fx, encompassing its primary natural sources, the metabolic biotransformation to fucoxanthinol (FxOH) and amarouciaxanthin A—metabolites whose bioactive properties significantly contribute to the observed in vivo effects—and the molecular mechanisms underlying the biological activities of Fx and its metabolites, with emphasis on their modulation of key intracellular signalling pathways involved in inflammation, lipid metabolism, and cell proliferation. Furthermore, it explores how targeted structural modifications may enhance the pharmacokinetic profiles and expand the therapeutic potential of Fx-based compounds, while highlighting promising strategies for their optimisation. By integrating insights from pharmacology, biochemistry, and synthetic chemistry, this work aims to guide future efforts in the rational design of marine-derived bioactive agents and underscores the value of marine biodiversity in therapeutic innovation.