The plastic waste crisis, particularly polyethylene terephthalate (PET) bottles, poses a serious environmental challenge with destructive impacts on ecosystems. In this regard, the present study aims to propose an innovative and sustainable solution by evaluating the reuse of PET bottles in the form of geocells for improving soil foundations. To achieve this, a laboratory-scale physical model was designed, and plate load tests were conducted on a shallow foundation placed on loose sand under two conditions: unreinforced and reinforced with geocells made from recycled PET bottles. The geometric parameters of the geocell layer, including height, embedment depth, and width, were investigated to analyze their effects on bearing capacity, initial stiffness, and load–settlement behavior. The results demonstrated that PET geocells significantly enhance the bearing capacity and initial stiffness of the foundation and improve stress distribution beneath the footing. In the optimum configuration, the ultimate bearing capacity increased by up to eight times compared with the unreinforced case. Increasing the geocell height improved load distribution and compressive resistance, while reducing the embedment depth facilitated more effective load transfer from the foundation to the geocell and then to the soil. Moreover, increasing the geocell width amplified the membrane effect and reduced stress concentration beneath the foundation. Overall, the use of PET bottles in geocell construction represents an economical and environmentally friendly method for improving the geotechnical performance of shallow foundations.