The bladder (from Old English blædre 'bladder, blister, pimple') is a hollow organ in humans and other vertebrates that stores urine from the kidneys. In placental mammals, urine enters the bladder via the ureters and exits via the urethra during urination. In humans, the bladder is a distensible organ that sits on the pelvic floor. The typical adult human bladder will hold between 300 and 500 ml (10 and 17 fl oz) before the urge to empty occurs, but can hold considerably more.

The Latin phrase for "urinary bladder" is vesica urinaria, and the term vesical or prefix vesico- appear in connection with associated structures such as vesical veins. The modern Latin word for "bladder" – cystis – appears in associated terms such as cystitis (inflammation of the bladder).

In humans, the bladder is a hollow muscular organ situated at the base of the pelvis. In gross anatomy, the bladder can be divided into a broad fundus (base), a body, an apex, and a neck. The apex (also called the vertex) is directed forward toward the upper part of the pubic symphysis, and from there the median umbilical ligament continues upward on the back of the anterior abdominal wall to the umbilicus. The peritoneum is carried by it from the apex on to the abdominal wall to form the middle umbilical fold. The neck of the bladder is the area at the base of the trigone that surrounds the internal urethral orifice that leads to the urethra. In males, the neck of the urinary bladder is next to the prostate gland.

The bladder has three openings. The two ureters enter the bladder at ureteric orifices, and the urethra enters at the trigone of the bladder. These ureteric openings have mucosal flaps in front of them that act as valves in preventing the backflow of urine into the ureters, known as vesicoureteral reflux. Between the two ureteric openings is a raised area of tissue called the interureteric crest. This makes the upper boundary of the trigone. The trigone is an area of smooth muscle that forms the floor of the bladder above the urethra. It is an area of smooth tissue for the easy flow of urine into and from this part of the bladder – in contrast to the irregular surface formed by the rugae.

The walls of the bladder have a series of ridges, thick mucosal folds known as rugae that allow for the expansion of the bladder. The detrusor muscle is the muscular layer of the wall made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. The detrusor muscle is able to change its length. It can also contract for a long time whilst voiding, and it stays relaxed whilst the bladder is filling. The wall of the urinary bladder is normally 3–5 mm thick. When well distended, the wall is normally less than 3 mm.

In males, the prostate gland lies outside the opening for the urethra. The middle lobe of the prostate causes an elevation in the mucous membrane behind the internal urethral orifice called the uvula of urinary bladder. The uvula can enlarge when the prostate becomes enlarged.

The bladder is located below the peritoneal cavity near the pelvic floor and behind the pubic symphysis. In males, it lies in front of the rectum, separated by the rectovesical pouch, and is supported by fibres of the levator ani and of the prostate gland. In females, it lies in front of the uterus, separated by the vesicouterine pouch, and is supported by the elevator ani and the upper part of the vagina.

The bladder receives blood by the vesical arteries and drained into a network of vesical veins. The superior vesical artery supplies blood to the upper part of the bladder. The lower part of the bladder is supplied by the inferior vesical artery, both of which are branches of the internal iliac arteries. In females, the uterine and vaginal arteries provide additional blood supply. Venous drainage begins in a network of small vessels on the lower lateral surfaces of the bladder, which coalesce and travel with the lateral ligaments of the bladder into the internal iliac veins.