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Several studies have shown an elevated amount of microplastics around dammed areas, indicating that the presence of in-stream barriers in riverine ecosystems could influence the fate and transport of microplastics. However, in-stream barriers have not been extensively investigated as potential causes of microplastic spatial heterogeneity. In this study, we measured microplastic concentration and assessed its ecological risk in rivers connected to the dam in the midstream Citarum region, West Java Province, Indonesia. Samples were processed using the digestion method using hydrogen peroxide, followed by visual counting and Fourier transform infrared spectroscopy (FTIR) validation. An average of 22.92 ± 19.94 MP particles per 100 g of dry sediment was found in midstream Citarum. We found a significant difference in microplastic concentration between the upstream and downstream of the first dam (P < 0.05). The river before the first dam (Saguling Dam) had the highest microplastic concentration, while after the first dam, microplastic concentration tended to decrease. The Potential Ecological Risk Index (PERI) in this study ranged from 11.2 (low risk) to 12.006 (very dangerous), while the Pollution Load Index (PLI) in the midstream Citarum River ranged from 3.82 (low risk) to 10.19 (high risk). This is the first baseline survey of microplastic pollution in midstream Citarum, which would improve our understanding of microplastic's ecological impacts in freshwater environments. [ABSTRACT FROM AUTHOR]