Ultraviolet (UV) radiation, particularly UV-B (280–315 nm) and UV-A (315–400 nm), constitutes a pervasive environmental stressor with far-reaching consequences for natural habitats. The progressive decline of stratospheric ozone, driven by anthropogenic halogenated compounds, has amplified surface UV-B irradiance across latitudes, intensifying ecological pressure on terrestrial, freshwater, and marine ecosystems. This review synthesizes current knowledge on the mechanisms by which UV radiation disrupts biological systems—spanning DNA photolesion formation, reactive oxygen species (ROS) production, photosynthetic inhibition, and endocrine disruption—and examines their cascading effects across ecological trophic levels. We survey impacts on aquatic primary producers, amphibian populations, soil microbial communities, forest canopies, polar ecosystems, and reproductive physiology of fauna. Particular attention is given to the synergistic interaction of UV exposure with climate warming, pollutants, and habitat fragmentation. Quantitative dose–response data, regulatory thresholds, and comparative ecosystem vulnerability indices are presented in tabular form. Protective biological mechanisms, including UV-absorbing pigments, DNA repair pathways, and behavioral photoavoidance, are critically assessed alongside the limitations of these adaptations under current and projected UV regimes. The review concludes with a synthesis of research gaps, monitoring priorities, and policy recommendations for mitigating UV-driven biodiversity loss in a changing climate.