Glioblastoma multiforme (GBM) is one of the most challenging diseases to treat in clinical oncology due to its high mortality rates and inefficient conventional treatment methods. Difficulties with early detection, post-surgical recurrences, and resistance to chemotherapy and/or radiotherapy are important reasons for the poor prognosis of those with GBM. Over the past few decades, magnetic resonance (MR) theranostics using magnetic nanoparticles has shown unique advantages and great promises for the diagnosis and treatment of cancers. Magnetic nanoparticles not only serve as "molecular beacons" to enhance tumor contrast in magnetic resonance imaging (MRI), but also serve as "molecular bullets" for targeted drug delivery, controlled release, and induced hyperthermia. Moreover, multiple functions of magnetic nanoparticles can be synergistically engineered into a single nanoplatform, making it possible to simultaneously image, treat, target, and monitor the targeted lesions. The multi-functionality of nanoparticles, also called nano-theranostics, gives rises to effective new approaches for combating GBM. In this work, recent research and progress concerning the applications of MR nano-theranostics on GBM using magnetic nanoparticles will be highlighted, focusing on topics such as diagnosis, therapy, targeting, and hyperthermia, as well as outstanding challenges for MR nanotheranostics in treating GBM. The conclusions are generally applicable to other types of brain tumors.