Thermosensitive hydrogels are very important biomaterials used in drug delivery systems (DDSs), which gained increasing attention of researchers. Thermosensitive hydrogels have great potential in various applications, such as drug delivery, cell encapsulation, tissue engineering, and etc. Especially, injectable thermosensitive hydrogels with lower sol-gel transition temperature around physiological temperature have been extensively studied. By in vivo injection, the hydrogels formed non-flowing gel at body temperature. Upon incorporation of pharmaceutical agents, the hydrogel systems could act as sustained drug release depot in situ. Injectable thermosensitive hydrogel systems have a number of advantages, including simplicity of drug formulation, protective environment for drugs, prolonged and localized drug delivery, and ease of application. The objective of this review is to summarize fundamentals, applications, and recent advances of injectable thermosensitive hydrogel as DDSs, including chitosan and related derivatives, poly(N-isopropylacrylamide)-based (PNIPAAM) copolymers, poly(ethylene oxide)/poly(propylene oxide) (PEO/PPO) copolymers and its derivatives, and poly(ethylene glycol)/ biodegradable polyester copolymers.