Polyethylene terephthalate (PET) has transitioned from a major source of plastic pollution to a strategic feedstock for creating novel and sustainable materials. This chapter presents a comprehensive overview of recycling and upcycling pathways that transform PET waste into functional ecomaterials such as adsorbents, membranes, catalysts, and photocatalysts for environmental and energy applications. Emphasis is placed on the mechanistic principles and efficiency of mechanical, chemical, and hybrid recycling, followed by advanced upcycling routes enabling molecular refunctionalization and nanostructured material design. PET-derived systems demonstrate high performance in pollutant adsorption, photocatalytic degradation, CO₂ reduction, and membrane-based separations, integrating sustainability with advanced functionality. Life-cycle assessments and circular economy analyses highlight their environmental and economic advantages compared to conventional materials. Overall, the chapter redefines PET as a versatile precursor for next-generation ecomaterials, illustrating how green chemistry, nanotechnology, and circular design converge to transform persistent polymer waste into high-value materials that promote environmental remediation and energy sustainability.