A ribozyme, or RNA enzyme, is an RNA molecule that can catalyze a chemical reaction. All ribozymes found naturally so far catalyze either their own cleavage or the cleavage of other RNAs. Some originate in the introns of RNA transcripts. The first ribozyme discovered was a self-splicing RNA from the ciliated protozoan Tetrahymena thermophila and was identified by Thomas R. Cech in the 1980s.

The existence of ribozymes disproved a common supposition in biological science, namely, that only protein molecules should be able to perform catalysis and functional operations in the cell. In reality, although proteins are much more chemically complex and ductile than nucleic acids, there is no basis for the claim that the latter cannot catalyze chemical reactions.

Although ribozymes are quite rare in the cell, their roles are sometimes essential to life. For example, the functional part of the ribosome, the molecular machine that translates RNA into proteins, is fundamentally a ribozyme.

According to the "RNA world hypothesis" of the origin of life, ribozymes may have been the first molecular machines used by early life. If this were the case, today's remaining ribozymes -- such as the ribosome machinery -- could be considered living fossils of a life based primarily on nucleic acids.

A recent test-tube study of prion folding suggests that an RNA may catalyze the pathological protein conformation in the manner of a chaperone enzyme.