A rare Ru-catalyzed highly selective synthesis of 3,4-dihydroisoquinolines or isoquinolines is accomplished via a redox-divergent hydrogen-retentive or hydrogen-releasing fashion. Notably, high cis-selectivity of 3,4-dihydroisoquinolines is achieved. Potential applications are shown by gram-scale reactions and very concise synthesis of N-containing polycyclic aromatic compounds. Primary mechanistic investigations indicate that the sequence of the major pathway involves Ru-catalyzed C-H activation, alkyne insertion, and subsequent 6π-electrocyclization.