Thiazolidine is a heterocyclic organic compound with the formula (CH₂)₃(NH)S. It is a 5-membered saturated ring with a thioether group and an amine group in the 1 and 3 positions. It is a sulfur analog of oxazolidine. Thiazolidine is a colorless liquid.

Derivatives, thiazolidines, are known. For example, the drug pioglitazone contains a thiazolidine ring. Another drug that contains a thiazolidine ring is the antibiotic penicillin.