The blood-brain barrier (BBB) is a physiological structure of the blood vessels in the brain. The BBB efficiently traps most therapeutic drugs in the blood vessels and stops them from entering the brain tissue, resulting in a decreased therapeutic efficiency. In this study, we developed BBB-stealth nanocomposites composed of iron oxide (Fe3O4) nanoparticles (NPs) as a safe nanocarrier for glioblastoma therapy. We showed the antitumor activity of Dox/alg-Fe3O4 NPs using in vitro and in vivo tests. We demonstrated that G23-alg-Fe3O4 NPs crossed the BBB and entered the brain. In situ glioblastoma tumor-bearing mice were used to successfully evaluate the antitumor activity of G23-Dox/alg-Fe3O4 NPs. Magnetic resonance imaging (MRI) and bioluminescence imaging (BLI) confirmed the BBB crossing. The BBB-stealth nanocomposites show great potential for a proof-of-concept clinical trial as a theranostics platform for human brain tumor therapy.