This study is for adults who had bladder cancer and had their bladder removed with surgery. To join, a person must have bladder cancer confirmed by a microscope. Their surgery must have been at least three weeks ago but not more than twelve weeks ago when they first join. They must have no signs of cancer left in their body, and their scans must show no spread of the disease. Tumor samples from the surgery must be available for a special blood test called the Signatera test. People cannot join if they have autoimmune diseases, lung inflammation, heart inflammation, active hepatitis B or C, or if they have had certain cancer treatments before. They must be at least 18 years old, not pregnant, and able to take care of themselves. Some people may join later if a blood test shows that cancer DNA appears in their blood after surgery. They must still have no signs of cancer on their scans, and their health must be safe for treatment with the study drug nivolumab.