A novel locally injectable, biodegradable, and thermo-sensitive hydrogel made from chitosan and β-glycerophosphate salt was prepared. It incorporated polyethylenimine (PEI)-modified super-paramagnetic graphene oxide (GO/IONP/PEI) as a form of minimally invasive treatment of cancer lesions by magnetically induced local hyperthermia. Doxorubicin (DOX) was mixed into the hydrogel which was pre-loaded on GO/IONP/PEI to create a drug delivery system DOX-GO/IONP/PEI-gel. In addition to the evaluation of in vitro and in vivo antitumor activities, the physicochemical properties, magnetic properties and DOX release profile of the DOX-GO/IONP/PEI-gel were determined. The aqueous solution of the hydrogel showed a sol-gel transition behavior depending on temperature changes. Magnetization loops indicated the super-paramagnetic properties of GO/IONP/PEI. Compared with free DOX, DOX-GO/IONP/PEI could efficiently pass through cell membranes, leading to more apoptosis and demonstrating higher antitumor efficacy on MCF-7 cells in vitro. Furthermore, DOX-GO/IONP/PEI-gel intratumorally injected (i.t.) showed high antitumor efficacy on tumor-bearing mice in vivo, with no obvious toxicity. The antitumor efficacy was higher when combined with an alternating magnetic field (AMF), showing that DOX-GO/IONP/PEI-gel under AMF has great potential for cancer magnetic hyperthermia therapy.