Background: One of the biggest global health issues and the main cause of intensive care unit (ICU) admissions is sepsis. Biomarkers can have a very vital role in sepsis diagnosis. The 16 kDa, 167-amino acid peptide leptin is mainly produced by adipocytes and has a role in controlling energy balance by telling the brain adipose tissue volume  in the body, which in turn controls how much food is consumed and how much energy is used. Leptin is also a cytokine that is involved in cytokine crosstalk and cell-mediated immunity. According to earlier researchs, sepsis patients' leptin level is elevated when compared to controls. Objective: Our study aimed to test the validity of serum leptin as a biomarker for sepsis diagnosis and to correlate between the level of leptin and sepsis prognosis and mortality. Methods: Twenty five critically ill patients diagnosed with sepsis were included in our study, ten healthy volunteers as a control group. The patients were admitted to the critical care department in Beni-Suef university hospital in the period from October 2023 to February 2024.Serum samples were collected from the studied patients on admission day (day 0), 2^nd day of admission (day 2) and 5^th day of admission (day 5), and only once in control group to correlate between leptin levels in patients and control group and to find the correlation between leptin and mortality. Results: Our study showed that leptin levels showed strong positive correlation with body mass index (BMI). No significant correlation was found between leptin in the studied group and control group on admission (day 0) while there was a highly significant correlation on day 2 and day 5. There was no significant difference in leptin levels between sepsis and septic shock patient on day 0, day 2 and day 5. There was a weak linear relation between leptin level with APACHE II & SOFA scores. There was non-significant difference in leptin level between survivors and non-survivors on day 0, 2 and 5. Conclusion: From our study we could suggest that leptin level in serum can be used as a good diagnostic marker in sepsis but not a good prognostic one.