The random disposal and incineration of plastic materials have caused a significant waste of resources and environmental pollution, which contradicts the recent emphasis on energy conservation and emission reduction. Carbon and hydrogen sources stored in plastic wastes have immense potential for the development of a carbon-neutral future. In this study, we use a two-step process for upcycling polyethylene terephthalate (PET), the most common polyester plastic, with methanol into high-value products, that is, lactic acid (LA) and 1,4-cyclohexanedicarboxylic acid (CHDA), using a commercial Ru/C catalyst. After the depolymerization of PET in a NaOH–methanol solution, the produced ethylene glycol can further react with methanol to obtain LA and hydrogen, and the hydrogen is then employed in the hydrogenation step to obtain CHDA in high yield. Notably, our method does not require an external supply of hydrogen gas (H2). This study reveals a new pathway for upcycling the two monomers from PET.