© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Transparent p-type semiconductors with wide-range tunability of the hole density are rare. Developing such materials is a challenge in the field of transparent electronics that utilize invisible electric circuits. In this paper, a CuI–CuBr alloy (CuI1−xBrx) is proposed as a hole-density-tunable p-type transparent semiconductor that can be fabricated at room temperature. First-principles calculations predict that the acceptor state originating from copper vacancies in CuBr is deeper than that in CuI, leading to the hypothesis that the hole density in CuI1−xBrx can be tuned over a wide range by varying x between 0 and 1. The experimental results support this hypothesis. The hole density in CuI1−xBrx polycrystalline alloy layers can be tuned by over three orders of magnitude (1017–1020 cm−3) by varying x. In other words, the p-type conductivity of the CuI1−xBrx alloy shows metallic and semiconducting properties. Such alloy layers can be prepared at room temperature without sacrificing transparency. Furthermore, CuI1−xBrx forms transparent p–n diodes with n-type amorphous In–Ga–Zn–O layers, and these diodes have satisfactory rectification performance. Therefore, CuI1−xBrx alloy is an excellent p-type transparent semiconductor for which the p-type conductivity can be tailored in a wide range.