Biopharmaceuticals, including proteins, DNAs, and RNAs, hold vast promise for the treatment of many disorders, such as cancer, diabetes, autoimmune diseases, infectious diseases, and rare diseases. The application of biopharmaceuticals, however, is limited by their poor stability, immunogenicity, suboptimal pharmacokinetic performance, undesired tissue distribution, and low penetration through biological barriers. In situ polymerization provides an appealing and promising platform to improve the pharmacological characteristics of biopharmaceuticals. Instead of the traditional "grafting to" polymer–biomolecule conjugation, in situ polymerization grows polymers on the surfaces of the biomacromolecules, resulting in easier purification procedures, high conjugation yields, and unique structures. Herein, this review surveys recent advances in the polymerization methodologies. Additionally, we further review improved therapeutic performance of the resultant nanomedicines. Finally, the opportunities, as well as the challenges, of these nanocomposites in the biomedical fields are discussed.