Optogenetic tools have revolutionized the study of receptor-mediated processes, but such tools are lacking for RNA-controlled systems. In particular, light-activated regulatory RNAs are needed for spatiotemporal control of gene expression. To fill this gap, we used selection to isolate a novel riboswitch that selectively binds the isoform of a stiff-stilbene (amino-SS)-a rapidly and reversibly photoisomerizing small molecule. Structural probing revealed that the RNA binds amino-SS about 100-times stronger than the photoisoform (amino-SS). and functional analysis showed that the riboswitch, termed Werewolf-1 (Were-1), inhibits translation of a downstream open reading frame when bound to amino-SS. Photoisomerization of the ligand with a sub-millisecond pulse of light induced the protein expression. In contrast, amino-SS supported protein expression, which was inhibited upon photoisomerization to amino-SS. Reversible photoregulation of gene expression using a genetically encoded RNA will likely facilitate high-resolution spatiotemporal analysis of complex RNA processes.