An ortho-alkenylation reaction of an aromatic aldimine with an internal alkyne is efficiently promoted by a cobalt catalyst generated from CoBr2, triarylphosphine, and iPrMgBr. The reaction takes place under mild room-temperature conditions to afford, upon acidic hydrolysis, a variety of ortho-alkenylated aro- matic aldehydes in moderate to excellent yields. The neighboring formyl and alkenyl groups in the product can be utilized as synthetic handles for the facile construction of indene and naphthalene carbocycles.