This study investigated the abundance of microplastics in surface water and sediments of the Opak River in Yogyakarta, Indonesia. Water and sediment samples were collected from six sampling points (P1-P6) chosen based on the accessibility and possible entry channels of microplastics. The samples were digested using 30% H₂O₂ using a 0.05 M Fe(II) catalyst at 70°C. The density separation of microplastics was performed using a saturated NaCl solution. The abundance, shape, and color of microplastics were observed using a microscope, and the polymers of microplastics were qualitatively analyzed using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR). The microplastics found in the surface water ranged from 4.15 to 12.3 particles/L, while in sediments, they ranged from 960 to 3080 particles/kg dry sediment. The shapes of microplastics were dominated by fragments, and other shapes such as pellet, foam, film, and fiber were also found. The colors included black, blown, transparent, red, green, and yellow, with black as the dominating color. Some polymers found in the microplastics were polystyrene, high-density polyethylene, polycarbonate, and nylon. The results of this study contributed to the advancement of the investigation of microplastics in river environments.