Intramolecular radical cation Diels–Alder reactions can be conducted under photocatalytic conditions using visible light irradiation. The photocatalyst system involves the use of a Ru(bpy)32+ chromophore and methyl viologen as a co-oxidant. These reactions enable the cycloaddition of substrates whose thermal Diels–Alder cycloadditions are electronically mismatched and thus require forcing conditions. Nevertheless, the radical cation cycloadditions can be conducted on gram scale using ambient sunlight as the only source of irradiation.