With the global surge in plastic consumption, sustainable recycling has emerged as a cornerstone of environmental stewardship. Polyethylene terephthalate (PET) is a leading player in the recycling revolution, yet ensuring the quality and authenticity of recycled PET (rPET) remains a critical challenge. This article presents a rapid, robust, and quantitative high performance liquid chromatography (HPLC) method for analyzing cyclic oligomers in rPET. This approach aligns with the Japanese Ministry of the Environment guidelines, enabling precise evaluation of rPET quality. The method leverages gradient elution and a practical calibration approach to deliver high throughput and accuracy, supporting the circular economy and the future of sustainable materials.