Herein we report the development of polyethylenimine-coated hollow manganese oxide nanoparticles for cancer targeted siRNA delivery and simultaneous magnetic resonance imaging (MRI). A mussel-derived adhesive moiety, 3,4-dihydroxy-L-phenylalanine (DOPA), was utilized as a robust anchor for the surface immobilization of polyethylenimine on the hollow manganese oxide nanoparticles. These nanoparticles were subsequently functionalized with anti-HER2 antibodies (Herceptin). We found that they efficiently delivered therapeutic siRNA into human breast cancer cells, and also generated strong positive T1 contrast on the MR image. These nanomaterials could be potentially utilized as multi-functional agents for cancer therapy using therapeutic siRNA and MRI-based diagnosis.