The fate and transport of Hg species in natural aquatic environment are strongly affected by photochemical transformation of Hg0, Hg2+, and MeHg. Migration of Hg is determined by its complexation with organic and inorganic ligands that are widely present in the water. The presence of dissolved organic matter (DOM) is closely related to photochemical reactions of Hg. DOM can strongly bind to mercury (e.g., Hg2+ and MeHg), thus affecting its speciation, mobility and toxicity, eventually dominating its bioavailability. This review summarizes extensive studies on photochemical behaviors of Hg including: (1) photo-oxidation; (2) photo-reduction; (3) photochemical methylation; and (4) MeHg photo-degradation. Photo-oxidation of Hg0 is mostly caused by oxidative free radicals (e.g., •OH, CO3•-, O3, and 1O2), while photo-reduction of Hg2+ is more complicated and it involves two pathways: (1) primary processes (direct photolysis of Hg2+ or ligand-metal charge transfer of Hg2+-DOM complex); and (2) secondary processes (reduction of Hg2+-DOM complex induced by free radicals derived from DOM photolysis). Photochemical methylation of inorganic Hg occurs as follows: (1) Hg2+ complexes with methyl donors (e.g., acetic acid, tert-butyl, alcohols, etc.) to form intermediates, followed by (2) an intramolecular methyl transfer. MeHg photo-degradation is the leading pathway for MeHg demethylation and it primarily proceeds via four different pathways. The information on DOM was also mentioned, but DOM is not the only factor that affects the photochemical behaviors of Hg. Other influencing factors such as: (1) pH value; (2) dissolved oxygen; (3) cations (Fe3+, K+) and anions (NO3-, HCO3-, CO32-, Cl-); and (4) suspended substance cannot be ignored.