The combination of chemotherapy and protein therapy holds immense promise for tumor treatment. However, conventional bolus formulation or simple co-administration fails to achieve maximized efficacy due to the distinct physiological and pharmacological properties of small molecular drugs and protein therapeutics. As such, developing a co-delivery system for optimal antitumor efficacy remains a great challenge. Herein, we achieve the synergistic co-delivery of human interferon-α2b (IFN) and doxorubicin (Dox) by the facile synthesis of a multifunctional and stimuli-responsive protein-drug-polymer (PDP) conjugate IFN-PolyDox-PEP. IFN-PolyDox-PEP is constituted of IFN, a polydisulfide tethering multiple copies of Dox (PolyDox), and a stealthy polypeptide (PEP) for long circulation. IFN-PolyDox-PEP self-assembles into nanoparticles of 122 nm in diameter and is able to release its therapeutic cargos in response to tumor microenvironment cues such as matrix metalloproteinase, acidic pH, and reducing glutathione. The in vivo synergistic effect of IFN-PolyDox-PEP is demonstrated by the superior antitumor efficacy as compared with the co-administration of IFN-polypeptide conjugate and free Dox. This work demonstrates the power of highly efficient chemistry in constructing well-defined PDP conjugates, which are promising hybrid macromolecules for various chemo-protein combination therapies.