We investigate the encapsulation of polymer-tethered nanoparticles (NPs) inside block copolymer (BCP) micelles, which turns to the mutual stabilization of BCP micelles and NPs by each other. It has been demonstrated, for the first time, that in the presence of cosolvent, which is selective for the core-forming block, the encapsulation of polymer-tethered NPs into the core of preformed BCP micelles is feasible. Furthermore, interestingly, in a certain range of cosolvent volume fraction, NP/BCP hybrid micelles comprising a single NP in the micelle core could be obtained. Significantly, in the presence of cosolvent the NP-loaded hybrid micelles remain intact, whereas the NP-free (empty) micelles undergo further transformations. The experimental observations are additionally supported by the results obtained from the molecular dynamic simulation. In particular, our modeling recognizes mechanisms for the formation of NP/BCP hybrids, which include a one-by-one adsorption of individual BCP chains onto the NP surface, fusion of NP with the preformed BCP micelles, or a combination of both mechanisms. The present work provides new opportunities for the fabrication of BCP-based hybrid nanomaterials having potential applications in drug delivery, diagnostics, catalysis, energy harvesting materials, optical devices, etc.