Cyclobutenone was employed as a dienophile in Diels−Alder cycloadditions, provide diverse and complex cycloadducts in good yields. Experimental outcomes indicated cyclobutenone to be more reactive than either cyclopentenone or cyclohexenone. In addition, cycloadducts bearing a strained cyclobutanone moiety were able to undergo regioselective ring expansions to produce corresponding cyclopentanones, lactones, and lactams, which are otherwise difficultly obtained by direct Diels−Alder reactions.