Conventional micelles of amphiphilic block copolymers (BCPs) disassemble into individual polymer chains upon dilution to a critical concentration, which causes the premature release of the encapsulated drugs and reduces the drug’s bioavailability. Here, by integrating the emerging metal–organic cage (MOC) materials with BCPs, we introduce a new type of composite micellar nanoparticles, block co-polyMOC micelles (or BCPMMs), that are self-assembled in essence yet remarkably stable against dilution. BCPMMs are fabricated via a stepwise assembly strategy that combines MOCs and BCPs in a well-defined, unimolecular core–shell structure. The synergistical interplay between the two components accounts for the particle stability: the MOC core holds BCPs firmly in place and the BCPs increase the MOC’s bioavailability. When used as nanocarriers for anticancer drugs, BCPMMs showed an extended blood circulation, a favorable biodistribution, and eventually an improved treatment efficacy in vivo. Given the versatility in designing MOCs and BCPs, we envision that BCPMMs can serve as a modular platform for robust, multifunctional, and tunable nanomedicine.