Acute kidney injury (AKI) is a serious complication in postoperative ICU patients. The incidence of AKI varies substantially based on the type of surgery and definition used. This study focuses on the incidence of AKI in postoperative ICU patients using full KDIGO criteria and related outcomes regarding to different types of surgery. Retrospective cohort study. Tertiary level university hospital, eight anaesthesiological/surgical ICUs, between 2016 and 2018. 6261 adult patients. Primary outcome was 28-day all-cause mortality in different stages of AKI according to complete KDIGO criteria. We found 3497 (55.9%) postoperative ICU patients with AKI. The severity distribution of AKI stage 1 to 3 was 19.7%, 28.4% and 7.8%, respectively, and 235 (4%) patients received RRT. The 28-day mortality was 3% (n = 205). Increasing AKI severity was associated with increased 28-day mortality when adjusted for other variables (AKI 2°: OR 2.81; 95% CI 1.55 to 5.24; p < 0.001 and AKI 3°: OR 11.37.; 95% CI 5.91 to 22.55; p < 0.001). Besides AKI stages 2 and 3, age (OR 1.02; 95% CI 1.01 to 1.04, p < 0.001), NYHA IV (OR 2.23; 95% CI 1.03 to 4.43, p = 0.042), need for surgical reintervention within 48 h (OR 2.92; 95% CI 1.76 to 4.72, p = 0.001), urgent surgery (OR 1.78; 95% CI 1.15 to 2.71, p = 0.01), emergency surgery (OR 2.63; 95% CI 1.58 to 4.31, p = 0.001), vascular surgery (OR 2.01; 95% CI 1.06 to 3.98, p = 0.033), and orthopedic and trauma surgery (OR 3.79; 95% CI 1.98 to 7.09, p < 0.001) versus cardiac surgery was significantly associated with increased risk for 28-days mortality in multivariate analysis. AKI based on full KDIGO criteria is very common in postoperative ICU patients and it is associated with stepwise increase in 28-days mortality. •Incidence & outcome of AKI classified by creatinine (SCr) and urine output (UO) criteria are unknown in surgical ICU patients•AKI occurred in nearly 56% of patients within the first week after ICU admission and is associated increased mortality.•The assessment of AKI based on SCr alone failed to identify AKI in many ICU patients.•The assessment of AKI based on both criteria, SCr and UO, increased the AKI incidence by 35%.•Adding the UO criteria unveiled a much higher number of patients with AKI in postoperative ICU patients.