Fibrous dysplasia (FD) is a common benign intra- medullary lesion of the skeleton characterized by proliferation of fibro-osseous tissue. It is viewed as a developmental abnormality that is typically seen in adolescents and young adults. It can present as monostotic (70% to 80%), polyostotic fibrous dysplasia (20% to 30%), McCune-Albright syndrome (2% to 3%), or Mazabraud’s syndrome. Monostotic fibrous dysplasia accounts upto 30% in the ribs. It is often asymptomatic and incidentally detected on radiographs. We report a case of a 35-year-old lady who presented with swelling on the right chest wall of 5-year duration and was increasing in size and painful for 8 months. Her chest radiograph showed a cystic mass of the 12th rib. On computed tomography scan, mass turned out to be focal lytic lesion of the 12th rib. Surgical resection of 12th rib was done. Histopathology of the lesion confirmed the diagnosis of FD. Although it is fairly common in ribs, no previous literatures on FD of 12th rib was found.