We investigated the efficiency of poly(methyl methacrylate)-b-poly(methacrylic acid) (PMMA-b-PMAA) diblock copolymers as reactive polymeric surfactants for the functionalization of ZnO nanoparticles (NPs) of diameters ranging from 20 to 80 nm. PMMA-b-PMAA with molar masses in the range of 20.000 and 30.000 g/mol and PMAA content between 3 and 30 mol % were prepared by reversible addition-fragmentation chain transfer (RAFT) radical polymerization. Scanning transmission electron microscopy (STEM) showed efficient coverage of the particle surface with a polymer layer and infrared (IR) spectroscopy provided evidence of interaction of the PMAA segment (anchoring chains) with the NP surface. As demonstrated by dynamic light scattering (DLS) and UV-vis spectroscopy, the amphiphilic PMMA-b-PMAA block copolymers prevented agglomeration of ZnO NPs to great extent and thus increased transparency of ZnO suspensions in tetrahydrofuran (THF) and PMMA/ZnO nanocomposites in the visible light region. We also demonstrated the importance of the length of PMAA segment for ZnO surface functionalization. Optimal UV-vis performance of suspensions of functionalized NPs in THF as well as of PMMA/ZnO nanocomposites was achieved with PMMA-b-PMAA block copolymers containing 3 and 15 mol % of anchoring PMAA segment.