Tuberculosis is a systemic disease which involves skeletal and articular system very rarely. Osteoarticular tuberculosis commonly occurs in the vertebral column and more rarely in the sacroiliac joints. In this study, we report a 44-years-old male patient with low-grade fever, malabsorption syndrome, abdominal and pelvic ascites and low-back pain, that underwent (18)F-FDG PET/CT for identifying the cause of signs and symptoms after a negative abdominal CT and negative thorax radiography. The study revealed increased tracer uptake at the peritoneal ascites and at the right sacroiliac joint in absence of bone alteration suggesting a sacroiliitis. Staining of the ascitic fluid was positive for acid-fast bacilli (Ziehl-Neelsen) and in the subsequent abdominal paracentesis Mycobacterium Tuberculosis was isolated; the final diagnosis was abdominal tuberculosis with a sacroiliac joint involvement. The patient started antitubercular therapy for 6 months and the clinical conditions were resolved, in particular both back pain and ascites disappeared.