External photocontrol over RNA function has emerged as a useful tool for studying nucleic acid biology. Most current methods rely on fully synthetic nucleic acids with photocaged nucleobases, limiting application to relatively short synthetic RNAs. Here we report a method to gain photocontrol over RNA by postsynthetic acylation of 2'-hydroxyls with photoprotecting groups. One-step introduction of these groups efficiently blocks hybridization, which is restored after light exposure. Polyacylation (termed cloaking) enables control over a hammerhead ribozyme, illustrating optical control of RNA catalytic function. Use of the new approach on a transcribed 237 nt RNA aptamer demonstrates the utility of this method to switch on RNA folding in a cellular context, and underlines the potential for application in biological studies.