The present study aimed to elucidate the extent of microplastic (MP) contamination in the Guandu River basin. Samples were collected from surface waters using a Manta trawl at 11 sites in two seasonal periods. MPs abundance was 6.1 ± 4.9 particles m−3 in the cold-dry season, and 2.3 ± 1.1 particles m−3 in the warm-rainy season. Four shapes and 12 colors were identified, indicating a high degree of particle diversity. Fragments were the most common, followed by fibers. The frequent colors were black, blue and transparent. Eleven plastic polymers were identified using micro-Fourier transform infrared spectroscopy analysis; the dominant ones were polyethylene and polypropylene. Artificial cellulose particles (including the semisynthetic polymer rayon) were also detected in high relative abundance. No significant difference in MP abundance was found between seasons and limnic conditions. There was a significant trend toward higher abundance of MPs near areas with greater urban influence and lower abundance in reservoirs surrounded by less urbanized areas (upstream river). Therefore, MPs appeared to be influenced by land use and occupation, with higher abundance in more urbanized areas, suggesting an urban–rural effect.