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Abstract RNA exhibits remarkable capacity as a functional polymer, with broader catalytic and ligand‐binding capability than previously thought. Despite this, the low side chain diversity present in nucleic acids (two purines and two pyrimidines) relative to proteins (20+ side chains of varied charge, polarity, and chemical functionality) limits the capacity of functional RNAs to act as environmentally responsive polymers, as is possible for peptide‐based receptors and catalysts. Here we show that incorporation of the modified nucleobase 2‐thiouridine (2sU) into functional (aptamer and ribozyme) RNAs produces functionally inactivated polymers that can be activated by oxidative treatment. 2‐thiouridine lacksthe 2‐position oxygen found in uridine, altering its hydrogen bonding pattern. This limits critical interactions (e. g., G−U wobble pairs) that allow for proper folding. Oxidative desulfurization of the incorporated 2‐thiouridine moieties to uridine relieves this inability to fold properly, enabling recovery of function. This demonstration of expanded roles for RNA as environmentally responsive functional polymers challenges the notion that they are not known to be redox‐sensitive. Harnessing redox switchability in RNA could regulate cellular activities such as translation, or allow switching RNA between a “template” and a “catalytic” state in “RNA World” scenarios or in synthetic biology.