Nanomedicines are deemed as the most promising treatment modality for malignant cancers. Particularly, cancer nanomedicines based on synthetic polypeptides have gained interest because they possess excellent safety, unique hierarchical structure, and tailorable functionalities to suit for delivery of diverse drugs including synthetic drugs, peptides, proteins, and nucleic acids. A few polypeptide-based nanoformulations (e.g., NK105, NC6004, NK911, CT2103) are under phases I-III clinical investigation for treating patients with advanced solid tumors. In recent years, progress has been made in the development of robust and high drug loading, tumor-targeting, membrane-disrupting, and stimuli-sensitive nanomedicines from de novo functional polypeptides, which afford not only better safety and reduced adverse effects, but also further improved anticancer efficacy over clinical formulations. Moreover, virus-mimicking vehicles have been devised from polypeptides for efficient nonviral delivery of highly potent peptides, proteins, and nucleic acids, greatly advancing biotherapy for cancers. In this Perspective, we highlight the state-of-the-art design and fabrication of cancer nanomedicines based on synthetic polypeptides and, at the end, give our viewpoints on their future development for targeted cancer therapy and potential challenges for clinical translation.