<p>This study investigated microplastic (MP) contamination in freshwater and marine ecosystems in Bangladesh, focusing on the Buriganga River and the Bay of Bengal. Water and fish samples were collected, digested with H₂O₂, and analyzed using microscopy and micro-Fourier transform infrared spectroscopy for visual inspection and MP characterization. Microplastics were significantly more prevalent in the freshwater system and its fish compared to the marine environment (<i>p</i> ≤ 0.05), with variations observed across different depths. Fish, particularly those in the benthopelagic zone, showed substantial MPs contamination, predominantly in the form of fibers, fragments, and foam. Transparent MPs dominated in water samples, while colored MPs are more prevalent in fish. Polycarbonate, nitrile, and polypropylene were the major polymers identified in the water and fish samples. A significant positive correlation (<i>r</i> = 0.65, <i>p</i> ≤ 0.05) between MPs in water and fish suggests ingestion of MPs from their habitat.</p>