Laminaria japonica (L. japonica), a widely cultivated marine macroalga, has gained substantial attention in human nutrition due to its rich composition of carbohydrates, essential minerals, and vitamins. As one of the primary polysaccharides in L. japonica, fucoidan has demonstrated notable biological activities, including antiinflammatory, antioxidant, antimicrobial, antiviral, cardiovascular protective effects, modulation of gut microbiota, and improvement of metabolic disorders, thereby holding significant potential in food and healthcare applications. Despite these recognized benefits, systematic reviews addressing the extraction and purification methodologies, structural elucidation, compositional analysis, and associated bioactivities of L. japonica-derived fucoidan remain limited. This review comprehensively examines current advancements in fucoidan extraction and purification techniques, discusses its structural characterization, and synthesizes recent findings on its multifaceted biological activities to explore its potential and significance in human health.