Background: Microplastics (MPs) have emerged as a growing research focus in aquatic environments, particularly within freshwater aquaculture systems in recent years. However, the presence and characteristics of MPs in Iran's freshwater aquaculture systems remain unexplored. Objectives: To address this gap, this study aims to quantify and characterize MPs in freshwater aquaculture systems across two regions: Mazandaran and Golestan provinces in Iran. Methods: 30 samples of Oncorhynchus mykiss from each area collected from a pool in the fall in 2024. The MPs were extracted from the gastrointestinal tract, gill and liver and analyzed for abundance, shape, size and polymer compound. Their polymer compositions were determined using a Fourier transform infrared spectroscopy (FTIR). Results: The results showed that the digestive tract had the highest abundance of MPs (26±11 particles/g) and the liver had the lowest abundance (2±0.2 particles/g). The average MPs observed in all tissues studied was 83.11±92.5 particles. The highest abundance was fiber MPs (92.96%) and the lowest abundance was fragment (7. 04%). The results showed that the 300-600 µm group had the highest abundance (31.37%), while the 3000-5000 µm group had the lowest abundance (5. 88%). Black color had the highest abundance (54.9%) and pheasant color had the lowest abundance (1.96%). The polymer composition of MPs included polyethylene (57.14%), polyethylene terephthalate (23.81%), polypropylene (9.52%), and polyester (9.52% PS). Conclusion: Investigating the input of MPs into RAS facilities through the weathering of its plastic components and evaluating non-plastic alternatives for these components (such as natural biofilters), as well as techniques for removing MPs from the system, is of great importance to minimize the presence of these contaminants in RAS and their impact on global food security.