膀胱癌 泛指各种出自膀胱的恶性肿瘤，也就是有异常细胞大量增殖而不受管制。膀胱是贮存尿液的中空器官，外壁主要由肌肉构成，位于下腹部。最常见的膀胱癌细胞来自膀胱内面黏膜表皮，正式名称为移行上皮细胞癌(TCC)。

罹患膀胱癌最主要的危险因子是来自基因的影响，另外吸烟、长期接触某种染料（含苯胺（aniline）成份者，如纺织厂员工就可能接触到）、汽油或其他化学物质者也有较高的风险。有动物实验显示老鼠长期大量服用一种人工甜味剂"糖精"（沙卡林/saccharin）可导致膀胱癌，但人体尚未有相同的报告；另一种更常用的人工甜味剂阿斯巴甜（aspartame）则没有类似报告。

Bladder cancer is any of several types of cancer arising from the epithelial lining (i.e., the urothelium) of the urinary bladder. Rarely the bladder is involved by non-epithelial cancers, such as lymphoma or sarcoma, but these are not ordinarily included in the colloquial term "bladder cancer." It is a disease in which abnormal cells multiply without control in the bladder.