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Background: Cardiac surgery-associated acute kidney injury (CSA-AKI) is prevalent and increasingly reported. Its diagnosis traditionally follows the Kidney Disease: Improving Global Outcomes (KDIGO) AKI criteria. However, little evidence supports its appropriateness for cardiac surgery patients, particularly regarding the subtle serum creatinine change (ΔSCr) that defines mild AKI. Objectives: The purpose of the study was to investigate the ΔSCr threshold specific to CSA-AKI and compare its impact on CSA-AKI incidence and prognosis with the KDIGO AKI criteria threshold. Methods: A 10-year coronary surgery cohort with serial perioperative SCr measurements was retrospectively analyzed. The relationship and prognostically significant threshold of 48-hour absolute ΔSCr with 30-day mortality were explored using multivariate restricted cubic spline analysis and receiver-operating characteristic curve analysis. AKI incidence and prognostic value were compared between adopting the KDIGO or new thresholds. Results: Among 37,706 patients, 20,290 (53.8%) developed KDIGO-defined AKI. For stage-1 AKI (18,835, 49.9%), the majority (75.2%) were solely attributed to the KDIGO absolute criterion (48-hour ΔSCr ≥0.3 mg/dL). The 48-hour ΔSCr threshold associated with an adjusted odds ratio ≥1.00 for 30-day mortality was 0.549 mg/dL. A similar threshold (0.553 mg/dL) was also identified based on the Youden index cutoff. Applying the 0.55 mg/dL threshold to define stage 1 CSA-AKI, the overall and stage-1 CSA-AKI incidence decreased to 21.0% and 17.2%, with 27.7% of the stage-1 CSA-AKI solely attributed to the new criterion. The prognostic value for AKI defined by this new threshold was significantly higher than the KDIGO criteria. Conclusions: A cardiac surgery-specific ΔSCr threshold in defining AKI was notably higher compared with the current general AKI definition.