A domino Bi‐catalysed C−N/C−S bond formation of N‐sulfonylaziridines is developed with 1,4‐dithiane‐2,5‐diol to give 3,4‐dihydro‐1,4‐thiazines at room temperature. The use of Bi(OTf)3 as a catalyst, atom economy and regioselectivity are the important practical features.magnified image