Arsenic(As) is a notoriously toxic pollutant of health concern worldwide with potential risk of cancer induction, but meanwhile it is used as medicines for the treatment of different conditions including hematological cancers. Arsenic can undergo extensive metabolism in biological systems, and both toxicological and therapeutic effects of arsenic compounds are closely related to their metabolism. Recent studies have identified methylated thioarsenicals as a new class of arsenic metabolites in biological systems after exposure of inorganic and organic arsenicals, including arsenite, dimethylarsinic acid(DMAV), dimethylarsinous glutathione(DMAIIIGS), and arsenosugars. The increasing detection of thiolated arsenicals,including monomethylmonothioarsonic acid(MMMTAV), dimethylmonothioarsinic acid(DMMTAV) and its glutathione conjugate(DMMTAVGS), and dimethyldithioarsinic acid(DMDTAV) suggests that thioarsenicals may be important metabolites and play important roles in arsenic toxicity and therapeutic effects. Here we summarized the reported occurrence of thioarsenicals in biological systems, the possible formation pathways of thioarsenicals, and their toxicity, and discussed the biological implications of thioarsenicals on arsenic metabolism, toxicity, and therapeutic effects. Arsenic (As) is a notoriously toxic pollutant of health concern worldwide with potential risk of cancer induction, but meanwhile it is used as medicines for the treatment of different conditions including hematological cancers. Arsenic can now Extensive metabolism in biological systems, and both toxicological and therapeutic effects of arsenic compounds are closely related to their metabolism. Recent studies have identified methylated thioarsenicals as a new class of arsenic metabolites in biological systems after exposure of inorganic and organic arsenicals, including arsenite, dimethylarsinic acid (DMAV), dimethylarsinous glutathione (DMAIIIGS) , and arsenosugars. The increasing detection of thiolated arsenicals, including monomethylmonothioarsonic acid (MMMTAV), dimethylmonothioarsinic acid (DMMTAV) and its glutathione conjugate (DMMTAVGS), and dimethyldithioarsinic acid (DMDTAV) suggests that thioarsenicals may be important metabolites and play important roles in arsenic toxicity and th Here we summarized the reported occurrence of thioarsenicals in biological systems, the possible formation pathways of thioarsenicals, and their toxicity, and discussed the biological implications of thioarsenicals on arsenic metabolism, toxicity, and therapeutic effects.