In eukaryotes and archaea, an exosome is a protein complex of exoribonuclease enzymes that is responsible for the degradation of non-functional mRNA transcripts. In bacteria it is replaced by a simpler-in-structure, but homologous in function, protein complex called a degradosome.

Exosomes, as the name suggests, have an exonucleolytic function, meaning they degrade mRNA from the end of the molecule (in this case the 3' end) rather than a site-specific region within the molecule. Exosomes are said to have degradative activity in the 3' to 5' direction.