In recent years, microplastics (MPs) contamination has been emphasized as one of the most global ecological problems. Bottom sediments were identified as one of the major sinks of MPs, while river-lake systems - as the main transport routs of MPs into the coastal seas and the World Ocean. Thus, the MPs determination in freshwater sediments is an important task for identifying sedimentation processes and contamination sources. Density separation is commonly used to extract anthropogenic particles from bottom sediments. The proposed method is dedicated to determining the abundance of MPs in freshwater organic-rich sediments. Repeated wet peroxide oxidation was applied to digest organic matter that can hamper the microplastic determination in a sample. Potassium formate with a density 1.5 g/mL was used as a separation solution. This method is intended to identify various types of MPs (fibers, films, fragments and beads) ranging in size from 0.174 to 5 mm. The effectiveness of the MPs extraction with potassium formate was 98.0 ± 2 % (р = 0.05; n = 16), while the extraction of MPs when analyzed under a microscope was 92.0 ± 4% (р = 0.05; n = 20).