30S is the smaller subunit of the 70S ribosome of prokaryotes. It is a complex of ribosomal RNA and ribonucleoproteins that functions in mRNA translation. It includes the 16S ribosomal RNA. It has about 21 basic proteins.

Note: Sometimes the '30s is used as shorthand for the 1930s, the 1830s, or other such decades in various centuries – see List of decades