Background: Cytoreductive surgery was developed as a treatment for peritoneal carcinomatosis. However, this surgery is associated with important complications. The present study aimed to assess the relationship between lactacidemia and the rate of associated complications during the immediate postoperative period in the intensive care unit (ICU) in patients undergoing cytoreductive surgery. Methods: This was a retrospective observational study. A total of 57 patients underwent cytoreductive surgery. All patients were admitted to the ICU immediately after the surgery. Data on lactic acid levels at the time of admission and discharge from the ICU were collected. Postsurgical complications that occurred during the ICU stay were recorded according to failure-to-rescue analysis and their severity stratified according to the Clavien-Dindo classification. Results: The lactic acid levels at admission to the ICU were significantly higher in patients who developed complications, with an almost tripled unadjusted relative risk (2.9, 95% CI: 1.6, 5.3), than in those who did not develop complications for the lactacidemia threshold established in the cumulative sum curve graphs. After adjustment for confounding effects, the relative risk became even higher (3.1, 95% CI: 1.8, 3.6). Lactic acid levels were still significantly higher in this group at the time of discharge from the ICU. Conclusions Serum lactate level is a risk factor for postoperative complications in patients undergoing cytoreductive surgery for peritoneal carcinomatosis. This study suggests that the risk of developing severe complications almost triples with a lactic acid level of 2.5 mmol/L or higher at the time of admission in the ICU.