Every year millions of bottles of polyethylene terephthalate (PET) are discarded into the environment, when in fact these materials have a high potential to be recycled and/or beneficially reused. This study aims to evaluate the mechanical behavior of a clayey soil mixed with 10%, 20%, and 30% of fine crushed PET and with 3% and 5% of PET flakes. These new materials can possibly be used in myriad applications in geotechnical engineering. Triaxial tests at 50, 150, and 300 kPa confining stresses were performed to obtain the strength parameters of the soil-PET mixtures. The results show that, overall, the strength improvements of the mixtures were considerable; the friction angle had a substantial enhancement, while the cohesion slightly decreased with the addition of the PET. The soil strength parameters were highly influenced by the percentage of fine crushed PET and PET flakes added to the mixture. Improvements also depended on the confining level to which the samples were submitted. This new geotechnical material can be used in landfills, as well as for slopes stabilization and other geotechnical works, contributing to a better understanding and interpretation of the behavior of soil reinforced with PET material and giving an environmentally friendly end to this residue.