A general methodology towards stable high-performance photocontrollable organic thin-film transistors (OTFTs) by using photochromic spiropyrans (SP) combined with polymethyl methacrylate (PMMA) as a photoactive gate dielectric is reported. Capacitance and mobility transitions are demonstrated theoretically and experimentally when the SP molecules undergo their documented reversible photoisomerization: This universal strategy of incorporating molecular functionalities into OTFTs offers attractive new prospects for the development of molecular devices with tailored electronic and other properties.