An unprecedented C–H mononitration of indolines either at the –C₅ or –C₇ positions under mild condition is reported here. The roles of multiple weak interactions and factors such as steric factors, electronic effects, cation–π interactions, and solvent polarity were established, and we achieved a 100% regioselective electrophilic aromatic (EArS) nitration using Cu(NO₃)₂ or AgNO₃.