Novel biodegradable thermosensitive triblock copolymers of poly(D,L-3-methylglycolide)-block-poly(ethylene glycol)-block-poly(D,L-3-methylglycolide) (PMG-PEG-PMG) have been synthesized. Ring-opening polymerization of D,L-3-methyl-glycolide (MG) initiated with poly(ethylene glycol) (PEG) and Ca[N(SiMe3)2]2(THF)2 provided triblock copolymers with alternating lactyl/glycolyl sequences of controlled molecular weight, low polydispersity index and uniform chain structure. At relatively low temperatures (≈10 °C) these copolymers formed clear solutions in water up to high concentrations (50 wt.-%). Depending on molecular mass ratios of PMG and PEG blocks, a sol-gel transition or an increase in viscosity without gel formation was observed upon increasing the temperature of the aqueous solutions. The temperature-induced gelation was ascertained by rheology and dynamic differential scanning calorimetry (DDSC).