Abstract: The total mercury (THg) and methylmercury (MeHg) distributions in surface sediment in Dalian Bay and the factors driving the distributions were investigated in July 2016. The results indicated that the mean THg and MeHg concentrations in surface sediment were (79.5±47.1) ng/g and (0.53±0.26) ng/g, respectively. The methylation rate (MeHg%) in surface sediment was 0.39%-1.46%, and the mean was 0.74%. The THg and MeHg concentrations in surface sediment decreased gradually from the inner bay to the outer bay, but the opposite trend was found for the MeHg%. The MeHg% was also sensitive to mercury loading caused by aquaculture activities. The MeHg concentration significantly correlated with geochemical parameters, and the THg concentration and redox conditions appeared to be the major factors driving the MeHg concentration. The MeHg% significantly negatively correlated with the Hg/C ratio, and might be related to the proportion of bioavailable mercury in the sediment. The MeHg% did not significantly correlate with the total organic carbon or total sulfur contents, but the effects of organic matter and sulfur on mercury bioavailability should not be ignored. The results indicate that mercury bioavailability may play an important role in the production of MeHg in surface sediment in Dalian Bay and that decreasing mercury and organic matter inputs will proportionally decrease the MeHg concentrations in the sediment.