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Gold nanoclusters (AuNC) with a precise number of Au atoms feature unique properties for imaging applications, including size dependent photoluminescence, very high photostability, and large Stokes shift. However, their low colloidal stability often leads to aggregation and fusion into larger gold nanoparticles (AuNPs), hindering the wider use of AuNC. Here, we report the first successful encapsulation of NIR emitting AuNC in polymer nanoparticles (PNPs). The AuNC, with 25 gold atoms and stabilized by 6-mercaptohexanoic acid (MHA), were encapsulated in PNPs with ca. 50 nm in diameter, obtained by miniemulsion polymerization of butyl methacrylate (BMA) and varying amounts of methyl methacrylate (MMA). While thermo-initiation led to aggregation of AuNC into plasmonic (non-emissive) AuNPs, the use of photo-initiation effectively preserved the AuNC inside the PNPs. By encapsulating a large number of AuNC, we obtained PNPs with high brightness in the NIR, both by one- and two-photons excitation. [Display omitted] [ABSTRACT FROM AUTHOR]