Selective alkylation of indazoles is still a highly challenging topic in chemistry and the synthesis of important molecules. Herein, a novel highly selective N2-alkylation of indazoles with diazo compounds is described in the presence of TfOH. Unlike the traditional metal- and base-catalysed version, this protocol highlights the regioselectivity of alkylation of indazoles and a metal-free catalysis system, affording N2-alkylated products in good to excellent yields with high regioselectivity (N2/N1 up to 100/0) and excellent functional group tolerance. Furthermore, a gram scale synthesis was conducted successfully to give rise to the corresponding products. Mechanistic studies through control experiments provide plausible mechanistic proposals.