Thioproline is a nonproteinogenic amino acid with the formula (CH2SCH2NHCH)CO2H, although it crystallizes as the zwitterion (CH2SCH2NH+2CH)CO−2.

It consists of a 1,3-thiazolidine ring ( (CH2SCH2NHCH2)) substituted with a carboxylic acid.

It is synthesized by reaction of formaldehyde and cysteine.

[3] It occurs in nature, but rarely.

[4] It forms a coordination complex with cobalt.