Multiple myeloma is a hematological malignancy characterized by a clonal proliferation of plasma cells in the bone marrow. Extramedullary dissemination of multiple myeloma is uncommon. Only in rare cases, the malignant plasma cells of multiple myeloma had infiltrated the lung parenchyma. In this case report, we presented a case of multiple myeloma with lung plasmacytoma, in a 42-year-old patient, hospitalized for pain and infiltrative mass in the right lung. The results of his initial evaluation with computed tomography and positron emission tomograpy scanning, resembled lung tumor with bone metastasis. Surprisingly, biopsies from lung tumor and bone metastasis, revealed malignant plasma cells. We found M spike in protein electrophoresis and lambda monoclonal band in immune electrophoresis. A bone marrow biopsy evaluation was done and justified multiple myeloma diagnosis before hematology referral. Multiple myeloma diagnosis at the age of 42 is quite rare.