Objective To investigate the predictive performance of serum high mobility group protein B1 (HMGB1) combined with forkhead box O1 (FOXO1) levels for patients with type 2 diabetes mellitus (T2DM) and concomitant diabetic nephropathy (DN). Methods Eighty⁃two patients with T2DM were selected as the research subjects. According to the presence of concomitant DN, T2DM patients were divided into DN group (46 cases) or non⁃DN group (36 cases). General data were compared between the two groups. Multivariate Logistic stepwise regression model was used to analyze the influencing factors for patients with T2DM and concomitant DN. Receiver operating characteristic (ROC) curve was plotted to analyze the predictive performance of serum HMGB1 combined with FOXO1 levels for patients with T2DM and concomitant DN. Results Fasting blood glucose level, HbA1c level, and 24⁃hour urine protein quantification, as well as levels of serum creatinine, urea nitrogen, HMGB1, and FOXO1 in the DN group were higher than those in the non⁃DN group, while the glomerular filtration rate was lower than that in the non⁃DN group (P<0.05). Multivariate Logistic stepwise regression analysis revealed that elevated levels of serum HMGB1 and FOXO1 were influencing factors for patients with T2DM and concomitant DN (P<0.05). Spearman correlation analysis indicated that serum HMGB1 and FOXO1 levels positively correlated with patients with T2DM and concomitant DN (P<0.05). ROC curve analysis depicted that the sensitivities, specificities, and areas under the curve of serum HMGB1 and FOXO1 levels for alone and jointly predicting T2DM patients and concomitant DN were 0.94, 0.72, and 0.98, respectively, 0.83, 0.75, and 0.92, respectively, and 0.92, 0.77, and 0.98, respectively. The combined predictive performance was superior to that of either marker alone (P<0.05). Conclusion Elevated serum HMGB1 and FOXO1 levels are independent risk factors for patients with T2DM and concomitant DN, and positively correlate with its occurrence. The combination of both markers provides superior predictive performance for patients with T2DM and concomitant DN.