Gold nanoclusters (AuNCs) feature size-dependent photoluminescence, with an extremely high photostability and a large Stokes shift. However, they can have limited colloidal stability and low luminescence quantum yields that hinder promising application in bioimaging, sensing and catalysis. Here we report the encapsulation of AuNCs with 25 gold atoms stabilized with 6-mercaptohexanoic acid ligands (Au25(MHA)18) into methacrylate polymer nanoparticles (PNPs) with c.a. 50 nm diameter. The time evolution of these structures in aqueous dispersion shows the formation of gold nanoparticles (AuNPs) with c.a. 4 nm diameter, leading to a 2-fold emission amplification (EA) of the AuNCs due to the field enhancement effect of the plasmon. The EA can be controlled by the annealing time of the PNPs in dispersion, the temperature and irradiation time. The optical properties of the PNPs are frozen upon drying because the diffusion of AuNCs inside the PNPs is negligible in the absence of solvent.