The CONSTITUTIVE PHOTOMORPHOGENIC 1-SUPPRESSOR OF PHYA-105 (COP1-SPA) complex is a central repressor of photomorphogenesis. This complex acts as an E3 ubiquitin ligase downstream of various light signaling transduced from multiple photoreceptors in plants. How the COP1-SPA activity is regulated by divergent light-signaling pathways remains largely elusive. Here, we reproduced the regulation pathway of COP1-SPA in ultraviolet-B (UV-B) signaling in vitro and determined the cryo-electron microscopy structure of UV-B receptor UVR8 in complex with COP1. The complex formation is mediated by two-interface interactions between UV-B-activated UVR8 and COP1. Both interfaces are essential for the competitive binding of UVR8 against the signaling hub component HY5 to the COP1-SPA complex. We also show that RUP2 dissociates UVR8 from the COP1-SPA4-UVR8 complex and facilitates its redimerization. Our results support a UV-B signaling model that the COP1-SPA activity is repressed by UV-B-activated UVR8 and derepressed by RUP2, owing to competitive binding, and provide a framework for studying the regulatory roles of distinct photoreceptors on photomorphogenesis.