Widespread microplastic contamination has been of growing concern worldwide and poses a growing threat to the marine environment, particularly the impact on the food chain. This study quantified the presence of microplastics in two bivalves of commercial interest at the Pak Kadae Estuary, Bandon Bay, Surat Thani province, Thailand: green mussel Perna viridis (L.) and cockles Tegillarca granosa (L.). Data collection was carried out over two seasons: dry season (April - May 2019) and wet season (June - August 2019). Microplastics were extracted using a 10 % potassium hydroxide (KOH) digestion method and then identified under a microscope. Results confirmed the presence of microplastics in both cockles and green mussels, during the dry season, with mean concentrations of 0.30±0.07 and 1.26±0.10 items/individual, respectively. During the wet season, the mean microplastic concentrations were 0.20±0.07 and 0.56±0.15 items/individual in the cockles and green mussels, respectively. There was a greater concentration of microplastics found in green mussels compared to cockles (P < 0.05) with no seasonal variation being observed. Five different shapes of microplastics, including fiber, fragment, pellet, rod, and film, were detected in the soft tissues of the bivalves. Fibers were the most common shape in green mussels (49 - 69 %) during both seasons. Cockles, in contrast, predominantly contained the pellet shape in the dry season (49 %) and fiber in the wet season (59 %). In addition, the microplastic concentrations were positively correlated to shell heights of the bivalves (P < 0.01). The number of microplastics recorded in the two commercially exploited species demonstrated the need for controlling plastic pollution in coastal ecosystems. HIGHLIGHTS Green mussels showed higher contamination with microplastics than blood cockles An average (Mean±SD) of 0.28±0.05 to 2.14±0.36 items/individual in cockles and 0.65±0.13 to 3.43±0.21 items /individual in mussels were obtained Microplastic fibers were the dominant shape found in shellfish Blue microplastics was the most frequently found in green mussels whereas the black color dominated in cockles GRAPHICAL ABSTRACT