The global prevalence of plastic use has raised concerns regarding microplastic pollution, which is widespread in water bodies around the world and poses significant risks to aquatic life. However, uncertainties still exist regarding the amount, types, and chemical composition of microplastics, particularly in tropical urban rivers. This study provides scientific novelty by offering the first comprehensive assessment of the abundance, distribution, types, and chemical makeup of microplastics in both the surface water and sediment of the urban tropical Karang Mumus River in Samarinda City, East Kalimantan, Indonesia—an area previously unexplored in this context. Microplastics were present in all surface water and sediment samples collected from six different sampling stations, with an average of 3.61 ±1.26 particles/L in surface water and 1222.22 ±308.80 particles/kg in sediment. The average microplastic concentrations observed in this study fall within the range reported in other urban river systems. No clear pattern was identified concerning the concentration of microplastics along the river from upstream to downstream. Microplastic fibers, fragments, and films were frequently detected, with films being the most common type found, followed by fibers and fragments in both surface water and sediment. In the surface waters, most particles were identified as polyethylene, followed by polystyrene, nylon, olefin fiber, and polypropylene. In the sediments, polyethylene was also the most common, followed by polypropylene, polystyrene, olefin fiber, and polyvinyl chloride. This research advances our understanding of microplastic pollution in under-studied tropical urban river systems and serves as a baseline for future studies and mitigation strategies.