<p>Microplastics (MPs) and their contamination in various environments, including soils, have already arisen as an alarming environmental threat, for which the civil engineering community has very limited awareness. In this study, soil contamination levels were briefly reviewed, and a contamination parameter, named as Microplastic Contamination Ratio (MPR), was introduced considering the civil/geotechnical engineering perspective. Five different fine-grained soils, four bentonites, and a kaolinite are selected and mixed with tire wear MPs at various MPR values to simulate contamination levels reported for different soils in the literature. The influence of MPs on the plastic limit (PL), liquid limit (LL), and plasticity index (PI) of five fine-grained soils was investigated through an extensive amount of fall cone testing. These consistency limits are not only among the fundamental parameters for the engineering classification of soils but are also known to be linked to their engineering behavior in various aspects. It was shown that even small amounts of MPs (e.g. MPR = 0.015%) can influence the slope of the flow lines of bentonites, therefore could considerably decrease the LL of contaminated bentonites (up to 20%). At critical contamination levels, termed as MPR_cr, the plasticity indices of the studied bentonites decreased by 20% to 24% relative to their uncontaminated states, which is defined as the critical plasticity index (PI_cr). It was also shown that as MPR was further increased, both the LL and PI of the same soils increased modestly, then remained at a threshold value. Consequently, six new parameters, including critical (MPR_cr) and threshold (MPR_th) contamination ratios, critical (LL_cr) and threshold (LL_th) liquid limits, critical (PI_cr) and threshold (PI_th) plasticity indices, are defined for the first time in the literature. Mineralogy and soil type were found to influence the MP effect on LL and PI of fine-grained soils due to the complex nature of soil-microplastic-water interaction.</p>