In the presence of a catalytic amount of KI combined with oxidant mCPBA, a convenient catalytic procedure is developed for the preparation of 3-selanylindoles from indoles and diselenides. In this protocol, KI is first oxidized by mCPBA into hypoiodous acid, which reacts with diselenide to cleave Se-Se bond. The in situ generated active electrophilic selenium species then reacts with indole, affording 3-selanylindole via an electrophilic substitution mechanism. This catalytic selenation of indoles has mild reaction conditions and is a simple procedure, which extends the synthetic application of KI in organic synthesis.