PLGA–PEG–PLGA (PPP) triblock copolymer is the most widely studied thermosensitive hydrogel owing to its non-toxic, biocompatible, biodegradable, and thermosensitive properties. PPP thermosensitive hydrogels are being investigated as in situ gels because, at a low temperature, PPP solutions with drugs can be injected at the target site, and converted into a gel without surgical procedures. To meet the requirements of different therapeutic applications, PPP hydrogels with different properties need to be synthesized. The adjustable properties include the sol–gel transition temperature, gel window width, retention time and drug release time. Furthermore, thermo- and pH-, thermo- and electro-, and thermo- and photo-dual sensitive hydrogels are needed for some special therapies. Thus, this review examines the methods of modification of PPP thermosensitive hydrogels used to obtain desired drug delivery systems with appropriate physicochemical and pharmaceutical properties.