In vertebrates the gallbladder (cholecyst, gall bladder, biliary vesicle) is a small organ that aids mainly in fat digestion and concentrates bile produced by the liver. In humans, the loss of the gallbladder is usually easily tolerated. The surgical removal of the gallbladder is called a cholecystectomy.

The gallbladder is a hollow system that sits just beneath the liver. In adults, the gallbladder measures approximately 8 centimetres (3.1 in) in length and 4 centimetres (1.6 in) in diameter when fully distended. It is divided into three sections: fundus, body and neck. The neck tapers and connects to the biliary tree via the cystic duct, which then joins the common hepatic duct to become the common bile duct. At the neck of the gallbladder is a mucosal fold called Hartmann's pouch, where gallstones commonly get stuck. The angle of the gallbladder is located between the costal margin and the lateral margin of the rectus abdominis muscle.

The different layers of the gallbladder are as follows: