A restriction enzyme is an endonuclease enzyme that cleaves DNA into small fragments at or near specific recognition sites that are known as restriction sites. Different restriction enzymes recognize and cut different DNA sequences.

Restriction enzymes are originally found in bacteria and archaea, where they provide a defense mechanism against invading viruses by selectively cutting up foreign DNA. Nowadays, these enzymes are routinely used in laboratories for DNA modification, serving as a vital tool in molecular cloning.