This is the first report about the ring-opening polymerization of thiolactide, which is the thioester analogue of lactide. The cyclic condensation of thiolactic acid gave rac-thiolactide, which underwent the ring-opening polymerization by a combination of thiol as the initiator and DBU as the catalyst. The polymerization was in equilibrium, showing that the monomer conversions were as low as 20% in solvents. The bulk polymerization at r.t. led the monomer conversions around 50%, and the molecular weights of the polymer products increased with decreasing the amount of the thiol initiator. The MALDI-TOF mass spectra revealed that the hexane-insoluble polythiolactide (Mn = 3000–3500, PDI = 1.3–1.6, 20–30%yield) had not only the linear but also the macro cyclic structures. A much weaker base, pyridine, worked as the catalyst, showing the lower activity but leading the polymerization to a more controllable fashion. DFT calculation suggested that rac-thiolactide has the smaller ring-strain than rac-lactide, which agreed with the lower polymerizability of rac-thiolactide. Polythiolactide showed the higher degradability for alkaline hydrolysis and UV photolysis than polylactide. Copolymerization of thiolactide with thioglycolide was also studied. Display omitted •Thiolactide, the thioester analogue of lactide, undergoes ring-opening polymerization.•Thiol-amine combinations conduct the ring-opening polymerization of thiolactide.•The ring-opening polymerization of thiolactide proceeds in an equilibrium manner.•Polythiolactide is more sensitive for alkaline hydrolysis and UV degradation than polylactide.