The Sundarbans, which is the largest mangrove forest in the world, are immensely threatened by plastic waste. For Sundarbans, plastic pollution is a relatively recent and serious issue. We investigated the quantitative assessment of microplastics (MPs) in surface waters, and sediments in the Sundarbans mangrove region of the northern Bay of Bengal, Bangladesh coast. The average concentrations of MPs observed during the pre-monsoon sampling period (surface water: 81,056 ± 8085 items/km2, sediments: 47 ± 11 items/kg) were higher than the post-monsoon (surface water: 73,722 ± 9096 items/km2, sediments: 41 ± 7 items/kg). MPs abundance was significantly higher in the downstream sampling station compared to the upstream. MPs were dominated by fibers in type, smaller dimensions (301–500 µm) in shape, black in color, and polyethylene terephthalate (surface water: 30%, sediments: 24%) in composition. Comprehensive research is strongly urged for plastic contamination controls and management of this crucial ecosystem.