The CRISPR system provides adaptive immunity against mobile genetic elements in prokaryotes, using small CRISPR RNAs that direct effector complexes to degrade invading nucleic acids(1-3). Type III effector complexes were recently demonstrated to synthesize a novel second messenger, cyclic oligoadenylate, on binding target RNA(4,5). Cyclic oligoadenylate, in turn, binds to and activates ribonucleases and other factors-via a CRISPR-associated Rossman-fold domain-and thereby induces in the cell an antiviral state that is important for immunity. The mechanism of the 'off-switch' that resets the system is not understood. Here we identify the nuclease that degrades these cyclic oligoadenylate ring molecules. This 'ring nuclease' is itself a protein of the CRISPR-associated Rossman-fold family, and has a metal-independent mechanism that cleaves cyclic tetraadenylate rings to generate linear diadenylate species and switches off the antiviral state. The identification of ring nucleases adds an important insight to the CRISPR system.