AbstractThe Cu‐based halide semiconductor CuGaI4 was prepared by a high‐temperature melting method. Optoelectronic characterization and density functional theory calculations of CuGaI4 reveal a direct bandgap of 2.9 eV. The corresponding UV photodetector (PD) based on CuGaI4 demonstrates excellent UV response and rapid response time. In addition, a flexible PD based on CuGaI4 is prepared, which also displays excellent photoresponse characteristics and mechanical stability. This work provides a systematic study of this novel Cu‐based halide semiconductor and demonstrates the great potential of CuGaI4 for future UV optoelectronic devices.