A simple cobalt–diphosphine catalyst has been found to efficiently promote intramolecular cyclization of ortho- cyclopropylvinyl- and cyclopropylidenemethyl-substituted benzaldehydes into benzocyclooctadienone and benzocycloheptadienone derivatives, respectively. This ring-opening hydroacylation likely involves aldehyde C–H oxidative addition, olefin insertion, cyclopropane cleavage by β-carbon elimination, and C–C bond-forming reductive elimination, as supported by mechanistic experiments and DFT calculations.