We report the formation and ultraviolet (UV) photodetection of single-crystalline spherical ZnO particles by pulsed laser irradiation of commercial ZnO nanoparticles in water. The phase and microstructure analysis before and after laser irradiation reveals a crystal size increase and shape transformation from irregular to spherical. Time-dependent laser irradiation confirmed that fusion is the reason for nanoparticle growth up to single-crystalline spherical particles. After rapid cooling, they maintain size and shape and possess unique optical and electrical properties. Because of the single-crystalline feature and smooth surfaces, high and selective sensing of ultraviolet light is observed.