Microplastics are widespread contaminants in soils and terrestrial ecosystems in many areas worldwide. In this study, we measured soil organic carbon (SOC) and soil organic matter (SOM) in microplastic-treated soils to determine if the presence of microplastics could affect the accuracy of carbon-based soil quality indicator measurements. Six different sizes and types of microplastics were selected, and six soil samples were used to evaluate the impacts. Treating soil with polyethylene and low-density polyethylene significantly increased SOC (p < 0.05) when measured with the modified Walkley & Black method; microplastic addition (0.01%, v/v) increased SOC by >40% compared to control organic carbon-poor soil (<10.0 g kg−1). We conclude that the microplastics can disrupt the accurate measurement of SOC. Likely, the physicochemical treatment used in the SOC measurement process can cause the organic compounds and/or carbon complexes to be extracted from microplastics, and this can affect the results. Considering that SOC is a main indicator for assessing soil quality and the global carbon cycle, overestimations caused by microplastic contamination should be further discussed to identify appropriate ways to deal with microplastics as a new carbon source in the environment.