A Lewis acid-catalyzed Friedel‒Crafts/Michael cascade reaction between N-dialkyl-3-vinylanilines of N-tosylaziridines has been developed for the stereoselective synthesis of tetrahydroisoquinolines. The reaction of using Gd(OTf)₃ as a Lewis acid catalyst was tolerant to both the variety of N-dialkyl-3-vinylaniline and N-tosylaziridine substrates and provided access to highly functionalized tetrahydroisoquinolines in typically high yields with moderate-to-excellent diastereoselectivities.