Previous reports have evaluated the prognostic value of serum beta-2 microglobulin (B2MG) level in patients with non-Hodgkin lymphoma. However, its role in predicting clinical outcome of patients with diffuse large B-cell lymphoma (DLBCL) in the rituximab era has not been extensively investigated. Here, we evaluated the prognostic value of B2MG and proposed a new prognostic model including B2MG for patients with DLBCL. A total of 274 patients with newly diagnosed de novo DLBCL were retrospectively analyzed. We defined the best cutoff value as 3.2 mg/L by using a receiver operating characteristic curve. Patients with a B2MG level ≥3.2 mg/L had significantly lower overall survival (OS) and progression-free survival than those with a B2MG level <3.2 mg/L (3-year OS, 50.9% vs. 89.4%, p < 0.001; 3-year progression-free survival, 45.3% vs. 79.7%, p < 0.001). Multivariate analysis showed that B2MG, age, performance status, and Ann Arbor stage were independent prognostic factors for OS. We developed a new prognostic model consisting of these four significant factors. We stratified patients into four-risk groups: low (L, 0 factor), low-intermediate (LI, 1-2 factors), high-intermediate (HI, 3 factors), high (H, 4 factors). This new prognostic model showed better risk discrimination compared with the National Comprehensive Cancer Network-International Prognostic Index (5-year OS: 100% and 23.4% vs. 100% and 27.1%, in L and H risk groups, respectively). Our study suggested that B2MG level is a significant prognostic factor in patients with DLBCL. A new prognostic index composed of age, performance status, stage, and B2MG could stratify the outcomes of patients with DLBCL effectively and appears to be a valuable risk model for these patients. Copyright © 2016 John Wiley & Sons, Ltd.