Platelet mass index (PMI) as a prognostic indicator in pediatric sepsis has not been previously reported. In this retrospective observational study, we evaluated PMI's performance as a prognostic indicator in children aged younger than 18 years with sepsis and septic shock in relationship with survival. Over 5 years, we collected data from 122 children admitted to our pediatric intensive care unit (PICU). PMI accuracy was assessed with sensitivity and specificity and its discrimination was assessed using the area under the receiver operating characteristic curve (AUC). Median PMI values on days 1 and 3 of PICU admission were lower among nonsurvivors. On day 1 of PICU admission, a cutoff PMI value of 1,450 fL/nL resulted in a sensitivity of 72% and a specificity of 69%, and the AUC was 0.70 (95% confidence interval [CI]: 0.55-0.86). Similarly, on day 3, a cutoff of 900 fL/nL resulted in a sensitivity of 71% and a specificity of 70%, and the AUC was 0.76 (95% CI: 0.59-0.92). Our exploratory study suggests that low PMI in children with septic shock is associated with increased mortality. Considering the PMI's fair performance, further studies should be performed to assess its clinical value.