Microplastic fibers (MPFs) from textiles contribute significantly to the microplastic (MP) load in many environmental matrices. MPFs have been shown to be readily ingested by a wide range of organisms representing multiple trophic levels and have been shown to negatively impact organisms. Most fate and effect studies to date rely on pristine reference MP materials that have limited relevance compared to the partially degraded MP particles and fibers typically present in the natural environment. The current study aimed to develop and validate a rapid method for generating environmentally relevant polyester (PET) MPF reference materials with controllable levels of degradation. Importantly, the method produced the same degradation products, terephthalic acid (TA) and ethylene glycol (EG), as those generated during natural UV (sunlight) exposure of PET. Alkaline hydrolysis provided linearly increasing degrees of degraded PET MPFs over just a few hours, with full decomposition into molecular fragments occurring after 3 hours. The extent of physical degradation was determined by scanning electron microscopy, while chemical degradation was quantified by measuring the production of TA and EG degradation products. The proposed accelerated hydrolysis degradation method is relevant for producing partially degraded PET MPF reference materials for use in fate and effect studies and has the potential for application to other polymer susceptible to hydrolysis.