Around 5% of plasma cell neoplasias are solitary plasmacytomas, a tumor that is fairly rare. The presence of a localized tumor composed of monoclonal plasma cells that are the same as those found in multiple myeloma and the absence of symptoms that would suggest a disseminated form are used to establish the diagnosis. The thoracolumbar spine is the area most affected. Costal origin is infrequently described. In our case, the patient manifested a right anterosuperior chest wall mass. Imaging showed a mass of tissue with a significant zone of osteolysis of the first rib and no chest wall infiltration. Blood protein immunoelectrophoresis disclosed a monoclonal kappa type IgG. Bence-Jones proteinuria was positive, further suggesting a plasmacytoma of the rib. A percutaneous needle biopsy for pathology study and immunohistochemistry enabled the diagnosis of costal plasmacytoma. Search for other localizations was negative and the diagnosis of solitary plasmacytoma was retained. The patient received radiotherapy and has remained in remission for over a year. The pathology and imaging findings are used to make the diagnosis. Radiotherapy is the therapy of choice, but it presents a risk of progression to other bone lesions, medullary plasmacytosis, and multiple myeloma. There are no known factors that predict systemic recurrence. Surveillance is essential on a regular basis.