NbCl5 serves as an efficient catalyst for the intermolecular [2+2+2] cycloaddition of alkynes and alkenes to afford 1,3-cyclohexadienes. A convenient system has been developed using excess alkene, which enables the reaction to proceed without using low-valent niobium reagents or any reductants. This catalytic system affords 1,3-cyclohexadiene with high chemo- and regioselectivity, in contrast to a previously reported system involving NbCl3(DME) as a low-valent niobium catalyst.