Abstract: The Arabidopsis thaliana protein UVR8 is a photoreceptor for ultraviolet-B, mediating UV-B-induced photomorphogenesis. Under light conditions devoid of UV-B, UVR8 is localized in both the cytoplasm and nucleus as homodimers. Absorption of UV-B promotes the nuclear accumulation and instant monomerization of the photoreceptor, then interacts with CONSTITUTIVELY PHOTOMORPHOGENIC 1 to orchestrate a series of genes essential for UV-B protection. Besides being a UV-B-specific photoreceptor, UVR8 plays an important role in the cell to mediate the whole-plant UV-B response. This review focuses on recent understandings of the structure and physiological function of UVR8 and molecular mechanism of UVR8-mediated UV-B signal transduction.