Solitary plasmocytoma is a rare tumor accounting for 5% of all plasma cell neoplasias. The diagnosis is based on identification of the localized tumor composed of monoclonal plasma cells identical to those observed in multiple myeloma, and absence of the signs in favor of a disseminated form. We report the case of a52-year-old man who presented a growth of the chest wall in the left axillary region. Imaging disclosed a mass of tissue with a large zone of osteolysis of the 6th rib and infiltration of the chest wall. Surgical biopsy for pathology study and immunohistochemistry enabled the diagnosis of costal plasmocytoma. Blood protein immunoelectrophoresis revealed a monoclonal kappa type IgG. Bence-Jones proteinuria was positive. Search for other localizations was negative and the diagnosis of solitary plasmocytoma was retained. Radiotherapy was delivered and the patient has remained in remission at one year. Costal localization is rare for solitary plasmocytoma. The diagnosis is based on imaging findings and pathology. Radiotherapy is the treatment of choice but with the risk of progression with other bone lesions, the development of medullary plasmocytosis and multiple myeloma. Factors predictive of systemic recurrence have not been identified. Regular surveillance is required.