Methylmercury (CH3Hg+, or MeHg) is the most poisonous form of mercury (Hg) because it can enter into human bodies through the consumption of Hg-contaminated fish and shellfish. A first step toward bioaccumulation of MeHg in aquatic foods is the methylation of inorganic mercury, a process that is predominantly mediated by anaerobic bacteria, such as sulfate reducing bacteria and iron reducing bacteria. Many researches have confirmed that microbial methylation of mercury is an intracellular reaction. Therefore, MeHg production rates are not only related to the presence and productivity of methylating bacteria and also the biouptake of Hg to these anaerobic bacteria. To understand the pathways of Hg biouptake is indispensable to elucidate the mechanisms of microbial methylation. In this review, we systematically evaluated the current state of knowledge regarding the four pathways of mercury biouptake, Mer-based transport system, passive diffusion, facilitated diffusion and active transport. In the future, facilitated diffusion and active transport of inorganic mercury to the cytoplasm of microbial cells should be emphasized.