Here we disclose a cobaloxime-catalyzed acceptorless dehydrogenative cyclization of o-teraryls and pyridinium salts. As for dehydrogenative cyclization of o-teraryls, in stark contrast to the established methods such as the Scholl or Mallory reactions, this protocol does not require any strong acids or oxidants, showing high atom economy and a broad substrate scope. It operates at near room temperature with light as the source of energy. Acid or oxidant-sensitive functional groups, such as 4-methoxyphenyl, unprotected benzyl alcohol, silyl ether and thiophene, are tolerated. Remarkably, aryls with electron-withdrawing groups and electron-poor heteroarenes, such as pyridine and pyrimidine, can also react. Preliminary mechanistic study reveals that hydrogen gas is released during the reaction, and both light and the cobalt catalyst are important for the dehydrogenation step.