The on-DNA synthesis of highly substituted cyclobutanes was achieved through a photocatalytic [2 + 2] cycloaddition reaction in aqueous solution. Readily available DNA-tagged styrene derivatives were reacted with structurally diverse cinnamates in the presence of an iridium-based photocatalyst, Ir(ppy)2(dtbbpy)PF6, to forge two new C(sp3)-C(sp3) bonds. This transformation was demonstrated to have excellent functional group tolerance and allowed for the facile installation of a variety of heteroaromatic substituents on a densely functionalized cyclobutane scaffold.