Water pollution due to the use of bright colors and patterns in the textile industry derived from hazardous chemicals can damage the environment. For this reason, special treatment is needed in waste processing to reduce water pollution, one of which is the photocatalytic method. ZnO nanoparticles have potential in photocatalyst applications for pollutant degradation because they are non-toxic, biocompatible, and cheap. This material can be obtained using the green synthesis method from pineapple peel extract used as a bioreduction. So, this research aims to determine the effect of ZnO nanoparticle catalyst mass on the degradation of methylene blue dye in photocatalytic activity under UV light irradiation. The results show that the size of the catalyst mass influences the ability to degrade methylene blue. In this case, 5, 8, and 12 mg catalyst masses could degrade dyes by 97.52%, 98.51%, and 99.82% within 180 minutes. Thus, ZnO nanoparticles effectively degrade methylene blue dye under UV irradiation.