Objective: This study sought to investigate the association between plasma cell-free hemoglobin (pHb) and mortality in sepsis or septic shock. Methods: We performed a retrospective analysis of the prospective collected data of patients with sepsis or septic shock. Patients were divided into 4 groups according to their pHb concentrations using a restricted cubic spline: group I (pHb ≤20 mg/dL), group II (20 < pHb ≤40 mg/dL), group III (40 < pHb ≤60 mg/dL), and group IV (pHb >60 mg/dL). The primary outcome was the 28-day mortality, and a multivariate Cox proportional hazard regression method was used for analysis. Results: A total of 372 patients were included in the analysis and the overall 28-day mortality rate was 16.7%. The median pHb concentration of the patients was 24.8 mg/dL (reference range, 0-5). The mortality rate did not increase in proportion to the pHb concentrations and was the lowest in group II (20.3%, 11.0%, 16.3%, and 26.7% in groups I, II, III, and IV, respectively). In the Cox proportional hazard regression analysis, group I was independently associated with 28-day mortality compared with group II (hazard ratio, 2.19; 95% confidence interval, 1.18-4.07). Group IV had a higher mortality rate compared to group II, but it was not statistically significant (hazard ratio, 2.17; 95% confidence interval, 0.85-5.56). Conclusion A low concentration of pHb (pHb ≤20 mg/dL) was associated with 28-day mortality in patients with sepsis or septic shock.