Bortezomib (BTZ) is an important boronate proteasome inhibitor that is widely used in cancer therapy. However, the clinical application of BTZ suffers from poor stability and serious adverse effects. Herein, we fabricated metal-polyphenol nanoparticles for the covalent encapsulation of BTZ. BTZ-encapsulated tannic acid (TA)-Fe nanoparticles can be prepared by mixing BTZ, TA, and ferric chloride owing to the formation of metal-polyphenol coordination interaction and dynamic boronate ester bonds. The BTZ-encapsulated TA-Fe nanoparticles (BTZ NPs) are stable in physiological environment (pH 7.4) with minimal drug leakage. However, BTZ NPs can be disassembled in an acidic environment. Therefore, BTZ can be rapidly released from BTZ NPs in an acidic environment (pH 5.0). More than 50% BTZ can be released from BTZ NPs after 8 h incubation at pH 5.0. BTZ NPs exhibited high cytotoxicity against human osteosarcoma Saos-2 cells and human multiple myeloma OPM-2 cells. The metal-polyphenol nanoparticles can be a promising nanoplatform for the delivery of BTZ with simultaneously enhanced therapeutic efficacy and reduced side effects.