Polyphenol-based hollow nanoparticles have exhibited intriguing properties in drug delivery, catalysis, and energy storage, and the open-mouthed (bowl-shaped) structure is supposed to improve the performance of the nanoparticles by increasing the accessibility of the inner surface and the cavity. Up to now, cationic polymers, metal ions, and sacrificial templates are generally applied in the preparation of the polyphenol-based hollow nanoparticle, and these polyphenols are usually confined to tannin acid and lignin. Seeking for the novel assembly strategy with a wide selection of polyphenolic compounds would open a new horizon for the design, preparation, and applications of polyphenol-based functional materials. In this study, polyphenol nanoparticles with bowl-shaped structure were efficiently prepared through the polymerization-induced self-assembly of tea polyphenols in a few minutes. The formation mechanism was proposed on the basis of a series of TEM images revealing the morphology evolution process. The nanoparticles with good stability in carbonization process are candidates for bowl-shaped carbon spheres. These nanoparticles exhibit potent radical scavenging capacity, anticancer activity, high adsorption capacity of guest molecules (DOX and methylene blue), and pH-dependent release property, with promising use as active delivery systems. Moreover, the present polyphenol assembly strategy provides inspiration for fundamental studies on structure–property–function relationship of polyphenol-based materials.