To determine the prognostic performance of soluble urokinase-type plasminogen activator receptor (suPAR) and quick Sequential Organ Failure Assessment (qSOFA) in predicting the 28-day mortality of sepsis patients admitted to the emergency department (ED). A prospective, single-center observational study was conducted between June 2018 and June 2019. In total, 175 patients with sepsis and septic shock admitted to the ED were enrolled based on the Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). We assessed the qSOFA score on ED admission and measured serum suPAR levels by quantitative enzyme-linked immunosorbent assay. Univariate and multivariate analysis was performed to identify predictors of prognosis. Kaplan–Meier survival curves and areas under the receiver operating characteristic (ROC) curve for 28-day mortality were calculated. We estimated category-free net reclassification improvement (NRI) when suPAR was added to qSOFA. Increased suPAR levels were significantly associated with 28-day mortality [1.74 (1.24–2.51) ng/mL in survivors vs. 1.34 (0.96–2.00) ng/mL in non-survivors, p = 0.011] and with sepsis severity [1.34 (0.99–1.98) ng/mL in sepsis vs. 1.74 (1.22–2.65) ng/mL in septic shock, p = 0.039]. The area under the curve (AUC) for the prediction of 28-day mortality was 0.646 (95% confidence interval (CI): 0.553–0.740) for suPAR, 0.832 (95% CI: 0.692–0.923) for qSOFA and 0.864 (95% CI: 0.802–0.928) for combined suPAR and qSOFA. Serum suPAR did not significantly increase the AUC of the basic qSOFA, but a model containing suPAR and qSOFA had a continuous NRI of 11% (95% CI: 3.5–18.5%; p = 0.004). Serum suPAR was associated with sepsis severity and 28-day mortality. Adding suPAR to qSOFA increased the ROC curve area and improved its discrimination, suggesting that this might be a useful tool in sepsis mortality prediction models.