<p><i>Alnus</i> <i>japonica</i>, a deciduous tree native to East Asia, has garnered significant attention due to its ecological and pharmacological properties. As a member of the Betulaceae family, this species contributes to forest stability through nitrogen fixation and soil improvement. In addition to its environmental importance, <i>Alnus japonica</i> has been investigated for its potential therapeutic benefits, owing to its rich bioactive compounds, including diarylheptanoids, flavonoids, tannins, and phenolic acids. Recent studies report its anticancer, antioxidant, anti-inflammatory, hepatoprotective, lipid-regulating, and other activities, supporting its relevance in traditional and modern medicine. These bioactive constituents modulate key cellular pathways, particularly those involved in oxidative stress and immune response. This review aims to present the first comprehensive assessment of <i>Alnus japonica</i>, offering an in-depth analysis of its chemical composition, bioactivities, and potential mechanisms of action.</p>