In vertebrates the gallbladder (also gall bladder, biliary vesicle or cholecyst) is a small organ where bile (a fluid produced by the liver) is stored and concentrated before it is released into the small intestine. Humans can live without a gallbladder. The surgical removal of the gallbladder is called a cholecystectomy.

The main purpose of the gallbladder is to store bile, also called gall, needed for the digestion of food. The gallbladder is part of the biliary system and serves as a reservoir for bile, which is produced by the liver. The liver produces the bile, which then flows through the hepatic ducts into the gallbladder. At any one time, 30 to 60 millilitres (1.0 to 2.0 US fl oz) of bile is stored within the gallbladder.

When food containing fat enters the digestive tract, it stimulates the secretion of cholecystokinin (CCK) from I cells of the duodenum and jejunum. In response to cholecystokinin, the gallbladder rhythmically contracts and releases its contents into the common bile duct, eventually draining into the duodenum. The bile emulsifies fats in partly digested food, thereby assisting their absorption. Bile consists primarily of water and bile salts, and also acts as a means of eliminating bilirubin, a product of hemoglobin metabolism, from the body

The bile that is secreted by the liver and stored in the gallbladder is not the same as the bile that is secreted by the gallbladder. During gallbladder storage of bile, it is concentrated by removal of some water and electrolytes. This is through the active transport of sodium ions across the epithelia of the gallbladder, which creates an osmotic pressure that also causes water and other electrolytes such as chloride to be reabsorbed.