Primary pulmonary diffuse large B cell lymphoma (PPDLBCL) is extremely rare, with fewer than 40 cases reported to date and a lack of systemic analysis. Herein, we present a case of PPDLBCL mimicking metastasis in a heavily treated patient with breast cancer. To our knowledge, this is the first reported case of PPDLBCL in a patient with breast cancer. A 66-year-old Chinese female diagnosed with breast cancer 7.5 years previously and multiple bone metastases 31 months later presented with a new-onset subpleural nodule in the inferior lobe of left lung detected by routine follow-up in November 2017. A 18F-fluorodeoxyglucose (FDG) positron emission tomography-computed tomography scan showed that the pulmonary nodule was hypermetabolic with a maximum standard uptake value of 14.9, consistent with lung metastasis in view of her history of breast cancer and multiple bone involvement. Surprisingly, pathologic investigation revealed primary lung DLBCL, staged IEA. Systemic chemotherapy with R-CDOP (rituximab, cyclophosphamide, vindesine, doxorubicin liposome, and prednisone) achieved complete remission with mild side effects. At the latest follow-up in August 2019, the patient had disease-free survival of 21 months. The findings from this case indicate that primary pulmonary lymphoma should be included in the differential diagnostic checklist of pulmonary occupancy, even in solid tumor patients treated with multiple modalities. When a newly developed lung nodule is identified in such patients, clinicians should not take for granted that it is lung metastasis. Pathology results are a prerequisite for making a correct diagnosis, choosing appropriate treatment, and improving patient prognosis.