This article introduces protein–polymer systems that serve as chemical reaction spaces at the nanoscale. Such polymeric nanoreactors accommodate enzymatic reactions either inside polymer supramolecular assemblies or at their interfaces with the environment, or through a combination of both. We provide an overview of polymer structures that include micelles, vesicles (primarily polymersomes, but also PICsomes and peptosomes), dendrimers, layer‐by‐layer capsules, and capsids—all of which can accommodate sensitive biomolecules to mimic natural systems and functions. This article further deals with the functionalization of polymer membranes and the analytical techniques that apply to nanoreactors. In the last section, we highlight the most relevant applications of such polymeric nanoreactors in the domains of medicine, ecology, and biotechnology.