To clarify whether microplastics contribute to elevated bioaccumulation of methylmercury (MeHg) in aquatic organisms, we studied the sorption pattern of MeHg on polystyrene beads (PBs) and evaluated MeHg accumulation, via uptake of MeHg-adsorbed PB, in the oyster Crassostrea gigas. MeHg-cysteine conjugates were added to seawater at 10, 100, and 1000 µg/L as Hg. Polystyrene beads (φ = 0.02, 0.2, and 2 µm) were immersed in the seawater for 24 h. The concentrations of total mercury (T-Hg) adsorbed onto the PBs were then measured using the reduction vaporization method. T-Hg concentrations for the PBs with diameters of 0.02, 0.2, and 2 µm were 10.6 ± 0.4, 1.8 ± 0.1, and 1.3 ± 0.1 ng/mg-PBs, respectively, when immersed in 2 mL of MeHg-added seawater (100 µg/L as Hg). Thus, the adsorption efficiency of MeHg onto PBs was higher in the presence of smaller diameter PBs. Next, 1 mg of PBs immersed in 2 mL of seawater containing 100 µg/L of MeHg for 24 h was added to an oyster tank containing 1 L of seawater. The T-Hg concentration of the oysters was measured after 6 h of exposure. No significant difference was found in the T-Hg concentration of oysters in the presence of PBs (0.30 ± 0.01 to 0.37 ± 0.05 ng/mg as dry weight) with MeHg and in the absence of PBs (0.36 ± 0.03 ng/mg as dry weight). Our results suggest that the presence of PBs in seawater has little effect on MeHg uptake by oysters.