A Rh-catalyzed intramolecular cascade decarbonylative cyclization reaction of ortho-formyl group-tethered alkylidenecyclopropanes (ACPs) has been developed, affording 2-methylindenes in moderate to good yields. The reaction proceeded through a decarbonylative generation of Rh–H species, intramolecular migratory insertion, β-carbon elimination, and a reductive elimination from a π-allylic rhodium intermediate on the basis of a deuterium-labeling experiment as well as other control experiments.