Background Primary breast diffuse large B-cell lymphoma (PB-DLBCL) is a rare subtype of non-Hodgkin lymphoma (NHL) with limited data on the clinical features and prognostic factors. Patients and Methods A consecutive cohort of patients with PB-DLBCL was retrospectively analyzed in our hospital from February 1997 through July 2018. The primary endpoint is overall survival (OS) contributing to any cause. Results A total of 76 patients were diagnosed with PB-DLBCL. The median age at diagnosis was 51 years (range: 25-80 years), with female prevalence (98.7%). Forty (52.6%) patients had right-sided breast involvement but no bilateral breast involvement at diagnosis. Overall, disease stages IE and IIE were seen in 55 (72.4%) and 21 (27.6%) patients, respectively. According to the stage-modified International Prognostic Index (IPI), 37 (48.7%) patients were classified in the very good risk group (IPI 0). Of the 72 patients available, the non-germinal center B-cell (non-GCB) subtype of DLBCL was observed in 66 (91.6%) patients. All patients received anthracycline-based chemotherapy, 56 (73.7%) with rituximab, 31 (40.8%) also with additional radiation therapy, and 14 (18.4%) patients received a prophylactic intrathecal injection. Seven (9.2%) patients had refractory disease. With a median follow-up of 6.8 years (range 0.4-25.0 years), 10 (13.2%) patients had a relapse in the central nervous system (CNS) site. The 5-year and 10-year OS of all the patients was 97.2% (95% CI: 99.3-89.5) and 84.8% (95% CI: 70.0-93.5), respectively. The median OS was not reached. The median progression-free survival (PFS) was 10.3 years for patients with PB-DLBCL(Figure 2B). The 5-year PFS of all the patients was 76.3% (95% CI: 64.6-84.6). Univariate analysis revealed several prognostic factors, including stage-modified IPI, breast surgery, refractory disease, and CNS relapse. Multivariate analyses produced two independent prognostic factors for patients with PB-DLBCL, including stage-modified IPI score (2-3 versus 0) (hazard ratio: 19.114, 95% CI 1.841 to 198.451, p=0.013) and CNS relapse (hazard ratio: 5.522, 95% CI 1.059 to 28.788, p=0.043). Conclusion In our cohort, PB-DLBCL clinical features are similar to prior literature reports. Stage-modified IPI score and CNS relapse were associated with overall survival.