Microplastic contamination in edible bivalves and their surrounding environments is increasingly concerning because it links environmental pollution to potential dietary exposure. This study examined microplastic abundance, particle characteristics, and polymer composition in wild oysters (Saccostrea commercialis), surface water, and sediment from two contrasting sites in Rayong Province, Thailand: marine-influenced Noen Kho (NK) and freshwater-influenced La Won Canal (LW). Sixty adult oysters and associated water and sediment samples were collected from eight stations. Microplastics were extracted using chemical digestion and density separation, characterized by shape and size, and a randomly selected subset was identified by micro-Fourier transform infrared (µ-FTIR) spectroscopy. Microplastics occurred in all matrices. Mean abundance was 4.13 ± 2.55 particles/g wet weight in oysters, 1.37 ± 0.28 particles/L in water, and 466.67 ± 199.76 particles/kg dry weight in sediment. Abundance in water and sediment was significantly higher at LW than at NK (p < 0.01), whereas oysters showed no significant between-site difference. Particles <250 μm predominated, with fragments and fibers as the major morphologies. Within the FTIR-analyzed subset, PP and PE/LDPE occurred more frequently at LW, whereas PET represented a larger proportion at NK. Under the mean per-capita consumption scenario, estimated annual dietary exposure ranged from 888 to 1009 particles/person/year and was substantially higher under a high-consumption scenario. These findings show that spatial patterns differed among matrices, identify oyster consumption as a potential dietary exposure pathway, and indicate that exposure estimates depend on the consumption rate applied.