Sepsis is associated with high morbidity and mortality, and short-term mortality remains above 30% despite relevant supportive and antibiotic treatments. The aim of this systematic review was to summarize and discuss the current evidence of the association of an increased number of circulating immature platelets with disease severity and mortality in patients with sepsis or septic shock. The review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement and was registered at the PROSPERO database (registration number: CRD42018104326). A systematic literature search was performed in PubMed, Web of Science, Scopus, and Embase on June 20, 2018, without time restrictions. The included studies were quality-assessed by the National Institutes of Health's Quality Assessment Tools. In total, 14 studies were included. The parameters used for the determination of platelet maturity were mean platelet volume, immature platelets fraction, reticulated platelet percentage, and absolute immature platelets count. Nine studies reported significantly increased immature platelet markers in nonsurvivors of septic shock compared with survivors, as well as in patients with severe sepsis or septic shock compared with patients without severe sepsis and septic shock. Six of these nine studies demonstrated that increased immature platelet markers were predictors of mortality and/or disease severity (area under the receiver operating curve: 0.599–0.886). This review suggests that an increased number of circulating immature platelets is associated with increased disease severity and mortality in patients with sepsis and septic shock. Larger studies are needed to confirm whether immature platelets should be routinely monitored to support the prediction of disease severity and mortality in septic patients.