Aquaporin-9 (AQP9) is a membrane-span transport protein expressed in the liver. It is located in the cytoplasm membrane of hepatic cells. In addition to water, it is also permeable to glycerol, urea, and other small solutes. Several evidences have revealed that AQP9 is involved in multiple physiological and pathological process of the liver. This paper summarized the expression of AQP9 in the liver and the effect on the physiological and pathological processes of the liver. AQP9 may be defined as a novel target for diagnosis and treatment of hepatic diseases.