Purpose. Sepsis and septic shock are the major causes of death in intensive care units. This study aimed to evaluate the clinical safety and efficacy of mesenchymal stem cells (MSCs) in sepsis and septic shock patients. Methods. Ten patients were enrolled in the study. Adipose-derived MSC infusions were given (1 × 106/kg, on the 1st, 3rd, 5th, 7th, and 9th days of therapy) together with standard therapy. Before the MSC applications, blood samples were collected for cytokine assessment (TNF-α, IFN-γ, IL-2, IL-4, IL-6, IL-10). The clinical and laboratory improvements were recorded and compared with control groups selected retrospectively. The clinical trial was registered on 16.03.2022 with the registration number NCT05283317. Results. In the study group, the ages of patients ranged from 22 to 68 years, and APACHE II scores ranged from 14 to 42. In the control group, ages ranged from 22 to 80 years and their APACHE II scores were between 14–35. The survival rate in the study group was 100% on the 14th day whereas it was 70% on the 28th day. A significant decrease in the SOFA score (adjusted), clinical, and laboratory improvements were observed during the MSC administration. However, no significant cytokine level changes were observed. In the control group, the survival rate of 20 patients was 70% on the 14th day, whereas 60% was on the 28th day. While deaths were observed in the control group in the first week of treatment, deaths in the MSCs group were observed between the 15th and 28th days. Conclusion. MSCs treatment may have a positive impact on the survival rates of sepsis during the early phase. However, further randomized controlled studies with a large group of patients are needed. Trial Registration. This trial is registered with ClinicalTrials.gov Identifier: NCT05283317.