The objectives of this study are to investigate the strength properties and permeability of soil specimens treated with microplastic at different concentrations and samples treated with both microplastic and plant roots. A clayey soil was treated with polyethylene terephthalate (PE-T) at a concentration range between 0.25 and 4% (by dry mass of soil). The findings revealed that true (undrained) cohesion is increased with the increasing amount of PE-T in soil. It was also found that soil treated with PE-T at concentrations of 1%, 2%, and 4% exhibited greater stress increments with strain values near 2%. As the concentrations of PE-T increase, the stress gradually increases and shows ductile behavior. The study also found that the shear strength significantly increased in the soil sample treated with both PE-T and roots. The PE-T and roots help to prevent particles from sliding over each other and improve the interlocking of soil grains. Moreover, the coefficient of permeability increases by 72% and 87% in the soil treated with 1% PE-T and 1% PE-T + roots, respectively, as compared with the clean soil (control). The higher increase in the soil sample with PE-T+ root is caused by the higher increase in void space which allows for higher volumetric expansion which eases the fluid flow. The current study demonstrates that the growth rate of roots gradually increases with the increase of PE-T in the soil samples as compared to clean soil. This is due to the ability of soil samples containing microplastics to hold more water. The microstructure of soil and PE-T was examined by images in the scanning electron microscope.