Herein, we describe a mild and efficient electrochemical method for cycloaddition reactions of alkene radical cations. Anodic oxidation of olefins produces electrophilic alkene radical cations, which further react with either diazo compounds in a [2 + 1] cycloaddition toward cyclopropane synthesis, or styrene derivatives in a [2 + 2] cycloaddition producing cyclobutanes. Both processes are green, metal- and catalyst-free, and scalable and tolerate a broad range of electron-rich olefins.