In this study, hydrothermal extraction experiments were performed in a semibatch apparatus at varying temperatures and different extraction times to determine the optimum operating conditions for the removal of oligomers from poly(ethylene terephthalate) (PET) resins. High-molecular-weight PET in pellet form (M w: 100 kDa) was used as the raw material. The results obtained suggest that hydrothermal treatment enables partial extraction of oligomers from PET resins. Using theoretical methods and the experimental data, we calculated the maximum reaction time at each temperature, considering the potential for application in plastic processing. A maximum extraction efficiency of 16% was achieved by hydrothermal extraction at 200 °C for 10 minutes. This novel technique is a promising tool for the development of a next-generation, low-oligomer, eco-friendly method in a short time.