The chemical compound 5-methyluridine (symbol m5U or m5U), also called ribothymidine (rT)[footnote 1], is a pyrimidine nucleoside.

It is the ribonucleoside counterpart to the deoxyribonucleoside thymidine, which lacks a hydroxyl group at the 2' position.

5-Methyluridine contains a thymine base joined to a ribose pentose sugar.

It almost universally occurs in position 54 (part of the T arm) of eukaryotic and bacterial tRNA, serving to stabilize the molecule.

The same "T-loop" motif occurs in many other forms of noncoding RNA such as tmRNA and rRNA.