Thiazolines (/θaɪˈæzoʊliːnz/; or dihydrothiazoles) are a group of isomeric 5-membered heterocyclic compounds containing both sulfur and nitrogen in the ring.

Although unsubstituted thiazolines are rarely encountered themselves, their derivatives are more common and some are bioactive.

For example, in a common post-translational modification, cysteine residues are converted into thiazolines.

Three structural isomers of thiazoline exist depending on the position of the double bond.

The amino acid cysteine is produced industrially from substituted thiazole.