The impact of the intentional selenium doping of CuI thin films is investigated concerning crucial crystalline, electrical and optical properties. For selenium contents in between 0.1 at.% and 1 at.%, the carrier density can be systematically adjusted by the selenium supply during growth between cm and cm while transparency and crystallinity remain unaffected. By temperature‐dependent Hall‐effect measurements, a carrier freeze out is observed and the binding energy of the selenium dopant is determined. The long‐term electrical stability in combination with cappings is significantly improved compared to undoped or oxygen doped CuI. However, for selenium contents exceeding 1 at.%, major crystalline changes are observed that are presumably correlated to a phase transformation. Transmission and electrical measurements suggest that the solubility limit of Se in CuI is about 1 at.% since a degradation of the transparency and decreasing free hole densities are observed for Se contents exceeding 1 at.%. Hence, the doping limit for Se in CuI corresponds to 1 at.%.