Spectral and photochemical properties of ZnO nanocrystal solutions containing free nitronyl nitroxide radical 2-(o-hydroxyphenyl)-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (Nit(o-OH)Ph) were studied. The ZnO photoluminescence quenching by Nit(o-OH)Ph was observed and analysis of the dependences of ZnO photoluminescence intensity on radical concentrations revealed the prevalence of a static mechanism for this process. The changes in spectral properties of colloidal ZnO nanocrystal solutions in the presence of a Nit(o-OH)Ph radical under illumination were studied. The nitronyl nitroxide was found to undergo photoinduced reduction to the corresponding hydroxylamine both via the excited-state interaction with ethanol and capturing of the ZnO conduction band electrons. Photoreduction of the radical was observed under illumination with light of λ > 460 nm in the presence of 4.2 nm ZnO nanocrystals, while no changes took place in the case of larger 4.8 nm ZnO nanocrystals. This pronounced size effect was interpreted in terms of much stronger spatial exciton confinement in the smaller ZnO nanocrystals favouring the irreversible electron transfer from the photoexcited radical.