Polymeric micelles are extensively used for the delivery of hydrophobic drugs, which, however, suffer from unsatisfactory drug loading, colloidal uniformity, formulation stability, and drug release. Herein, we demonstrate a convenient strategy to prepare micelles with ultrahigh drug loading via the incorporation of polymer-drug coordination interactions. An amphiphilic copolymer containing pendant phenylboronic acid as electron acceptor unit was synthesized, which afforded donor-acceptor coordination with doxorubicin to obtain micelles with ultrahigh drug loading (∼50%), nearly quantitative loading efficiency (>95%), uniform size, and colloidal stability. Besides, the encapsulated drug can be effectively and selectively released in response to the high reactive oxygen species levels in cancer cells, which potentiated the anticancer efficacy and reduced systemic toxicity. Apart from doxorubicin, the current platform could be extended to other drugs with electron-donating groups (e.g., epirubicin and irinotecan), rendering a simple and robust strategy for enabling high drug loading in polymeric micelles and cancer-specific drug release.