The low therapeutic efficacy of conventional cancer chemotherapy has been associated with an immunosuppressive tumor microenvironment (TME). Tumor-associated macrophages (TAMs), which display an M2-like phenotype, are abundant in many tumors and facilitate tumor growth and resistance to therapy. Here, we show that poly(L-arginine) (PLR), a cationic poly(amino acid) can induce the polarization of macrophages into the tumor-suppressive M1 phenotype, in vitro. Further, we demonstrate that hyaluronic acid (HA) and PLR-coated manganese dioxide (MnO2) nanoparticles (hpMNPs) display efficient anti-cancer effects by upregulating nitric oxide (NO) production. Surface modification with biocompatible HA reduced the cytotoxicity of the cationic PLR. Additionally, manganese ions released from these nanoparticles by the high concentrations of glutathione (GSH) in the TME increased iNOS expression level in macrophages and enhanced the performance of T1 weighted magnetic resonance imaging. Particularly, our results illustrate the therapeutic effects, such as growth inhibition and apoptosis of tumor cells, of hpMNP treated macrophages. Therefore, the newly designed multifunctional PLR-assisted MNPs may facilitate the polarization of M2 macrophages into the M1 phenotype, which can mediate NO-dependent anticancer immunotherapy.