Foamed plastic debris in aquatic systems has become one of the emerging global contaminants. In this study, the behavior of polystyrene foam (PSF) and microplastics (MPs) adhered on the PSFs were investigated on the Tuul River shore in Ulaanbaatar, the capital city of Mongolia. The micro-sized (<5 mm) PSF, which was the dominant PSF over 600 pieces in 100 m2, have accumulated along the shoreline of Tuul River. Carbonyl index (CI) was calculated to evaluate the surface oxidation of macro-sized (20–100 mm), meso-sized (5–20 mm), and micro-sized PSFs and confirm the relative aging depending on photodegradation. CI ranged from 0.00 to 1.09 in the sampled PSFs, whereby the degraded PSFs with high CI were distributed on the shore of downstream of sewer drainage. Micro-sized PSFs showed a wide range of CI and a relatively high average value of CI as compared to those of meso- and macro-sized PSFs. Most of PSFs aggregated with MPs and the adhered MPs have been ubiquitously detected from the surface of PSFs. Adhered micro-sized plastics explored from the surface of PSFs with various sizes, except for mega-sized (>100 mm) PSF, ranged from 5 to 141 items per piece of PSF fragment. The aggregates of PSFs and MPs were common status of PSFs during their transportation. The present findings, which indicated a high concentration of adhered MPs, raise an environmental concern about the widespread aquatic plastic pollution. Display omitted •Polystyrene foams (PSFs) are dominant plastic debris in the studied river shore.•Wide range of surface oxidation of PSFs can be related to their surface status.•Abundance of adhered microplastics on PSFs was different between size fractions.•PSFs have a great potential to transport various types of microplastics. Environmental plastics which have various adverse effects to the ecosystem services have potentials to interact microplastics from the environment and to form plastic aggregates affecting to aquatic ecosystems by their behaviors.