A new C-C bond formation strategy based on enantioselective radical alkylation of C(sp3)-H bonds via Co(ii)-based metalloradical catalysis has been demonstrated for stereoselective synthesis of chiral indolines. The Co(ii)-based system enables activation of aryldiazomethanes as radical precursors at room temperature for enantioselective intramolecular radical alkylation of broad types of C-H bonds, constructing 2-substituted indolines in high yields with excellent enantioselectivities. In addition to chemoselectivity and regioselectivity, this Co(ii)-catalyzed alkylation features tolerance to functional groups and compatibility with heteroaryl substrates. Detailed mechanistic studies provide insight into the underlying stepwise radical pathway.