In molecular biology, SNORD18 (also known as U18) is a non-coding RNA (ncRNA) molecule which functions in the modification of other small nuclear RNAs (snRNAs).

This type of modifying RNA is usually located in the nucleolus of the eukaryotic cell which is a major site of snRNA biogenesis.

Most of the members of the box C/D family function in directing site-specific 2'-O-methylation of substrate RNAs.

[1] In humans and Xenopus laevis there are three closely related copies of U18 (called U18A, U18B, U18C) which are encoded in introns of ribosomal protein L1.

Related snoRNAs in Arabidopsis thaliana and rice Oryza sativa have been named R63 and Z106 respectively.