Open-shell singlet diradical molecules have been widely investigated because they are key to understanding the nature of chemical bonds. We propose a new concept for reversible switching of diradical character—an index of the instability of chemical bonds—of a molecule by photochromic reaction. Photochromic diarylethene derivatives with various open-shell singlet diradical characters are theoretically designed, and their photochromic diradical character switching behaviors are clarified. These results contribute to designing highly efficient third-order nonlinear optical switching substances based on the correlation between the diradical character and second hyperpolarizability.