A highly efficient and convenient metal‐free, one‐pot synthesis of diversely substituted (tetrahydro)quinolines has been achieved through a three‐component assembly reaction of arenediazonium salts, nitriles, and styrenes. In sharp contrast to the prior works with the same reagent blend, the formation of N‐arylnitrilium intermediates from arenediazonium salts and nitriles was followed by reaction with styrenes, leading to 3,4‐dihydroquinolinium salts as a common intermediate. These could be further transformed to quinolines and tetrahydroquinolines depending on the reaction conditions. The advantages of this protocol include its simplicity, metal‐free and mild conditions, readily available starting materials, and good functional group tolerance.