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Microplastics (MPs), or plastic particles smaller than 5 mm, are emerging contaminants that potentially affect aquatic species and humans. More research has been undertaken in the marine environment; however, rivers that serve as a sink and pathways for plastic entering the ocean have received less attention. The current study investigated the presence of MPs in the sediments of the Cagayan de Oro River, one of the top plastic-emitting rivers listed in the Philippines. Sediment samples were collected from the six sampling stations from upstream to downstream using a Van Veen grab sampler. Through stereomicroscope and Fourier-transform infrared spectroscopy (FTIR), MPs were analyzed and categorized according to color, shape, size, and polymer type. All the sampling stations were contaminated with MPs ranging from 4.4 ± 2.57 to 33.3 ± 4.44 items/kg. The highest density of MPs was found close to the river mouth that drains into Macajalar Bay which suggests that riverine contributions are substantial contributory to MPs contamination in the marine environments. Prevalent types of MPs are blue-colored (41%), fiber-shaped (41%), 0.5–1.0 mm (43%), and polyethylene (43%) particles. Microplastic accumulation is generally higher downstream than upstream. [ABSTRACT FROM AUTHOR]