Fucoidan, a sulfated polysaccharide, is primarily obtained from marine organisms, particularly brown algae and certain invertebrate species. Structurally, it is characterized by repeating units of l-fucose linked via either α(1 → 3) or α(1 → 4) glycosidic bonds. Fucoidan has been reported to exhibit a broad spectrum of biological activities, including anticancer activities and antiviral activities against both the influenza virus and SARS-CoV-2. Moreover, its safety as an edible component has contributed to growing interest in its potential therapeutic applications. However, the structural heterogeneity of fucoidan—arising from variations in glycosidic linkages, sulfation patterns, and molecular weights depending on the biological source and extraction conditions—has hindered the identification of specific glycan motifs responsible for its biological activities. To address this issue, elucidation of the detailed structure–activity relationships (SAR) using structurally well-defined fucoidan analogs is highly warranted. In this context, we focused on three types of fucoidan categorized based on their linkage patterns: type I, composed of α(1 → 3)-linked fucose; type II, composed of alternating α(1 → 3) and α(1 → 4) linked fucose; and type III, composed of α(1 → 4)-linked fucose. A series of low-molecular-weight fucoidan analogs with well-defined glycan backbones and diverse sulfation patterns was synthesized accordingly. SAR studies were subsequently conducted to elucidate the key structural features governing their biological activities. In this chapter, we present our recent work on the precise chemical synthesis of structurally defined low-molecular-weight fucoidan analogs and SAR investigations focusing on their anti-proliferative effects against human cancer cells, inhibitory activity against influenza virus infection, and inhibitory activity against SARS-CoV-2 infection.