Microplastic (MP) pollution has emerged as a critical environmental concern, posing ecological risks across various aquatic systems. In this study, sediment samples from the Han River estuary (Vietnam) were analyzed during the 2023 rainy season and the 2024 dry season to evaluate the spatial and temporal variations in MP contamination. The characterization of MPs was conducted based on density, morphology (shape and size), color, polymer composition, and Pollutant Load Index (PLI). Results revealed the presence of MPs at all sampling sites, exhibiting clear seasonal and spatial variability. MP concentrations were substantially higher in the rainy season (7,295.3 MPs/kg) compared to the dry season (2,371 MPs/kg). Small fragments and fibers (50–150 µm) were predominant, indicating intensive degradation and fragmentation processes. White particles were most abundant during the rainy season, while green particles dominated during the dry season. Seventeen polymer types were identified in the rainy season and ten in the dry season, with polyethylene terephthalate (PET) being the most prevalent polymer (2,828.7 and 792 MPs/kg, respectively). The average PLI value of 2.9 placed the area in pollution group I-representing a low pollutant load. However, the persistence and potential ecological risks associated with MPs warrant further investigation. Future studies should aim to refine the classification of plastic polymers and support the development of comprehensive management strategies to regulate plastic usage in daily life and industrial sectors, thereby mitigating MP pollution in estuarine ecosystems.