Zinc oxide nanoparticles (ZnO NPs) were synthesized by a precipitation method from zinc nitrate hexahydrate and sodium hydroxide using high-temperature dyeing equipment. This synthesis was studied within the framework of the percent yield to determine the most favorable concentrations of the chemical reagents. The in situ functionalization of cotton textiles with ZnO NPs was carried out via an exhaust dyeing method using the previously determined favorable concentrations of zinc nitrate (2, 5, 8 and 10% on-weight-fabric) and sodium hydroxide (0.7, 1.7, 2.5 and 3.0 g/L). The results demonstrated that the ZnO NPs in powder form and those synthesized in situ on the fabric exhibited a wurtzite hexagonal crystalline structure. The powder displayed a star-like morphology with very pronounced sharp points, whereas the NPs grown on the fabric showed a similar morphology but with not very pronounced sharp points. For all concentrations of zinc nitrate studied, before and after 20 washing cycles, the cotton fabric functionalized with the ZnO NPs exhibited a UV protection factor between very good and excellent.