Microplastic (MP) contamination poses significant risks to ecosystems and human health. However, the absence of standardized protocols, detailed polymer identification, and sources identification hinders the development of targeted mitigation strategies, particularly in developing nations. There is a scarcity of comprehensive data on MP distribution, sources, and transport mechanisms in freshwater environments. This study aimed to fill these gaps by comprehensively characterizing MP contamination, elucidating distribution patterns, identifying sources, and assessing ecological risks in an urban river adjacent to a megacity. This was accomplished using stereomicroscopy, Fourier-transform infrared(FTIR) spectroscopy, and a range of risk assessment indices. The analyses revealed spatial variations in MP levels, ranging from 350 to 660 items/m3 across different sampling stations along the river. Analysis of variance(ANOVA) highlighted significant differences in the average number of MPs among the stations (F = 16.93, p ≪ 0.01), with station S3 exhibiting the highest count and station S4 the lowest. Factors such as point sources of domestic and municipal waste, as well as river navigation, likely contribute to these variations. The predominant types, colors, and sizes of MPs observed were fiber, transparent, and <0.5 mm, respectively. Notably, 80% of the MPs consisted of polyethylene (PE) and polypropylene (PP), commonly associated with land-based sources like packaging materials. Despite minor ecological risks indicated by ecosystem risk assessment indices such as the risk index(RI) and pollution load index (PLI), which recorded values of 9.04 and 1.87, respectively, the potential hazard index(PHI) rose to hazard category V, posing a substantial threat to the river ecosystem. PCA facilitated the identification of trends linked to specific pollution sources, while cluster analysis categorized MPs with similar characteristics, thereby enhancing the understanding of their distribution patterns. These findings provided novel insights into the pervasive presence and pathways of plastic pollution in developing nations, offering important considerations for international efforts to address public health and environmental challenges associated with MPs.