Peritoneal tuberculosis (TB) is a relatively uncommon presentation of extrapulmonary TB. Early diagnosis of peritoneal TB is difficult because of its nonspecific clinical manifestation such as abdominal pain, fever, or ascites. Especially early after surgery of abdomen or pelvis, these symptoms can be misdiagnosed as septic peritonitis. There are few reports of peritoneal TB as a postoperative complication of laparoscopic surgery. Here, we describe a first case of peritoneal TB after robot-assisted laparoscopic prostatectomy (RALP) with extended lymph node dissection. A 78-year-old man presented 25 days after this surgery with fever and abdominal distension. Ultrasonography and computed tomography (CT) revealed massive abdominal ascites. Ascites sample was cloudy, with increased white blood cells and normal creatinine level. No anastomotic leak was found. Bacterial infection of a lymphocele was considered, and cefmetazole 2 g/day for 3 days was prescribed. Despite antibacterial therapy, fever persisted. Polymerase chain reaction testing of ascitic fluid was positive for Mycobacterium tuberculosis. The patient was effectively treated with anti-TB therapy. This is the first report of peritoneal TB as a postoperative complication of RALP with extended lymph node dissection. His preoperative chest CT showed granular shadows in left upper lung, indicating his old asymptomatic TB infection. Flare-up of TB can happen even after robot-assisted laparoscopic surgery, which is minimally invasive. Peritoneal TB must be considered especially when there is unexplained ascites unresponsive to antibiotics.