A novel biodegradable temperature-responsive copolymer, 4-arm star-shaped poly(D,L-lactic-co-glycolic acid)-b-methoxy poly(ethylene glycol) (4sPLGA-mPEG), was synthesized via the arm-first method. The copolymer solution of various concentrations was prepared to form vaginal gels at body temperature, in order to encapsulate different anti-HIV drugs, anti-inflammatory drugs or contraceptive drugs. The 4sPLGA-mPEG block copolymer solutions were liquid at room temperature and only the 4sPLGA-mPEG block copolymer solutions with a copolymer concentration from 20 to 40 wt% show a sol–gel transition as the temperature was increased. The viscosity change associated with the sol–gel phase transition depended on the copolymer concentration and DL-lactide/glycolide (LA/GA) mol ratio. The in vitro and in vivo biodegradation and biocompatibility of a thermogelling polymeric material were examined in this study. The degradation of the copolymer gel, proceeded by hydrolysis of ester bonds, was followed by the erosion of the gel in a simulated vaginal fluid solution at body temperature for nearly one month. Mass loss and reduction of the molecular weight were detected. The LA/GA mol ratio was found to significantly influence the degradation profiles. The rapid in vivo gel formation was confirmed after subcutaneous injection of the copolymer solution into Sprague-Dawley (SD) rats. The in vivo degradation was slightly accelerated compared to in vitro hydrolysis, and the persistence time of the injected hydrogels in vivo was found to be tuned by the LA/GA mol ratio. MTT assay and histological observations were used to examine the copolymer solution. Both in vitro and in vivo results illustrate acceptable biocompatibility of our materials. Collectively, our results show that the 4sPLGA-mPEG block copolymer is a promising candidate as a novel vaginal gel.