We report the formation of a self-assembled hydrogel of graphene oxide nanosheets by physically crosslinking in a low concentration Pluronic solution without any chemical modification of GO. This hydrogel undergoes a sol-gel transition upon exposure to various stimuli, such as temperature, near-infrared light, and pH. Since the thermosensitive gelation occurs at near body temperature, this system can be applied as an injectable gel system. NIR laser irradiation can also trigger very rapid gel formation by photothermal effect. An in vivo mouse experiment confirms the stable gel formation after injection with no severe acute cytotoxicity.