This study demonstrates the sustainable synthesis of multifunctional CIS@MIL-101(Cr) composites for water treatment applications. The composites were prepared via hybridization of CuInS2 with MIL-101(Cr) resulting in the formation of CIS nanoplates incorporated into MIL-101(Cr). The composites exhibited high visible light photocatalytic activity due to their low band gap energy and were tested for tetracycline photodegradation achieving a degradation efficiency of 98.8%. The material showed high stability after four cycles, and the effects of reactive species on photodegradation were investigated. The kinetics and mechanism of the photocatalytic process were studied, and HPLC-MS analysis was conducted to identify intermediate products. These results demonstrate the potential of using waste PET to create new semiconductors for water pollution control, promoting a circular material pathway.