The study analyzed the grain-size distribution of surface sediments in the Chanthaburi coast of Thailand to understand the sedimentary environment and its changes over time. Different methods were used to study the sediment dynamics and identify source materials. The coast mainly consists of sand and silt, with poor sorting that worsens further from the shore. Sediment grain size b (3.25–4.5φ) is a key indicator of environmental changes. Sediments in the area mainly move northwestward and northeastward from sea to land. The study divided the area into three provinces with different sedimentary environments and material sources. The tropical estuaries in Thailand have stronger hydrodynamic conditions compared to temperate estuaries in eastern China, leading to differences in sediment transport trends.