Bladder cancer is the abnormal growth of cells in the bladder. These cells can grow to form a tumor, which eventually spreads, damaging the bladder and other organs. Most people with bladder cancer are diagnosed after noticing blood in their urine. Those suspected of having bladder cancer typically have their bladder inspected by a thin medical camera, a procedure called cystoscopy. Suspected tumors are removed and examined to determine if they are cancerous. Based on how far the tumor has spread, the cancer case is assigned a stage 0 to 4; a higher stage indicates a more widespread and dangerous disease.

Those whose bladder tumors have not spread outside the bladder have the best prognoses. These tumors are typically surgically removed, and the person is treated with chemotherapy or one of several immune-stimulating therapies. Those whose tumors continue to grow, or whose tumors have penetrated the bladder muscle, often have their bladder surgically removed (radical cystectomy). People whose tumors have spread beyond the bladder have the worst prognoses; on average they survive a year from diagnosis. These people are treated with chemotherapy and immune checkpoint inhibitors, followed by enfortumab vedotin.

Around 500,000 people are diagnosed with bladder cancer each year, and 200,000 die of the disease. The risk of bladder cancer increases with age and the average age at diagnosis is 73. Tobacco smoking is the greatest contributor to bladder cancer risk, and causes around half of bladder cancer cases. Exposure to certain toxic chemicals or the tropical bladder infection schistosomiasis also increases the risk.

The most common symptom of bladder cancer is visible blood in the urine (haematuria) despite painless urination. This affects around 75% of people eventually diagnosed with the disease. Some instead have "microscopic haematuria" – small amounts of blood in the urine that can only be seen under a microscope during urinalysis – pain while urinating, or no symptoms at all (their tumors are detected during unrelated medical imaging). Less commonly, a tumor can block the flow of urine into the bladder; backed up urine can cause the kidneys to swell resulting in pain along the flank of the body between the ribs and the hips. Most people with blood in the urine do not have bladder cancer; up to 22% of those with visible haematuria and 5% with microscopic haematuria are diagnosed with the disease. Women with bladder cancer and haematuria are often misdiagnosed with urinary tract infections, delaying appropriate diagnosis and treatment.

Around 3% of people with bladder cancer have tumors that have already spread (metastasized) outside the bladder at the time of diagnosis. Bladder cancer most commonly spreads to the bones, lungs, liver, and nearby lymph nodes. Tumors cause different symptoms in each location. People whose cancer has metastasized to the bones most often experience bone pain or bone weakness that increases the risk of fractures. Lung tumors can cause persistent cough, coughing up blood, breathlessness, or recurrent chest infections. Cancer that has spread to the liver can cause general malaise, loss of appetite, weight loss, abdominal pain or swelling, jaundice (yellowing of the skin and eyes), and skin itch. Spreading to nearby lymph nodes can cause pain and swelling around the affected lymph nodes, typically in the abdomen or groin.

Those suspected of having bladder cancer undergo several tests to assess the presence and extent of any tumors. The gold standard is cystoscopy, wherein a flexible camera is threaded through the urethra and into the bladder to visually inspect for cancerous tissue. Cystoscopy is most efficient at detecting papillary tumors (tumors with a finger-like shape that grow into the urine-holding part of the bladder); it is less efficient with small, low-lying carcinoma in situ (CIS). CIS detection is improved by blue light cystoscopy, where a dye (hexaminolevulinate) that accumulates in cancer cells is injected into the bladder during cystoscopy. The dye fluoresces when the cystoscope shines blue light on it, allowing for more accurate detection of small tumors.

Suspected tumors are removed during cystoscopy in a procedure called "transurethral resection of bladder tumor" (TURBT). All tumors are removed, as well as a piece of the underlying bladder muscle. Removed tissue is examined by a pathologist to determine if it is cancerous. If the tumor is removed incompletely, or is determined to be particularly high risk, a repeat TURBT is performed 4 to 6 weeks later to detect and remove any additional tumors.

Several non-invasive tests are available to support the diagnosis. First, many undergo a physical examination that can involve a digital rectal exam and pelvic exam, where a doctor feels the pelvic area for unusual masses that could be tumors. Severe bladder tumors often shed cells into the urine; these can be detected by urine cytology, where cells are collected from a urine sample, and viewed under a microscope. Cytology can detect around two thirds of high-grade tumors, but detects just 1 in 8 low-grade tumors. Additional urine tests can be used to detect molecules associated with bladder cancer. Some detect the bladder tumor antigen (BTA) protein, or NMP22 that tend to be elevated in the urine of those with bladder cancer; some detect the mRNA of tumor-associated genes; some use fluorescence microscopy to detect cancerous cells, which is more sensitive than regular microscopy.