An expedient synthesis of benzo[c][1,2]dithiol-3-ones via metal-free denitrogenative transannulation of benzotriazinones is developed, which represents the first example of acid-mediated heteroannulation of benzotriazinones. This newly discovered reactivity of benzotriazinones enables the streamline synthesis of diverse benzo[c][1,2]dithiol-3-ones in decent yields by using sodium sulfide as the sulfur source under simple reaction conditions.