Objective: Acute pancreatitis (AP) is a common digestive emergency with high mortality in severe cases, making early risk identification crucial. This study aimed to investigate the association between first-day serum albumin (ALB) levels upon intensive care unit (ICU) admission and in-hospital mortality in patients with AP, and to explore its potential non-linear nature. Methods: This retrospective study analyzed data from 728 adult AP patients admitted to the ICU for the first time between 2008 and 2019, sourced from the MIMIC-IV database. First-day ICU ALB levels and relevant clinical data were extracted. Multivariable logistic regression models were used to assess the linear association between ALB and in-hospital mortality. Generalized additive models (GAM) and two-piecewise linear regression models were employed to explore non-linear relationships and identify inflection points. Results: After adjusting for multiple confounders, each 1 g/L increase in first-day ICU ALB was associated with a 27% reduction in in-hospital mortality risk (OR 0.73, 95% CI 0.59–0.92, P=0.007). GAM analysis further revealed a significant non-linear relationship between ALB and in-hospital mortality, with an inflection point at 28 g/L. When ALB was ≤ 28 g/L, each 1 g/L increase was associated with a 56% significant reduction in mortality risk (OR 0.44, 95% CI 0.26–0.73, P=0.001); however, this association was not statistically significant when ALB was > 28 g/L. Conclusion: First-day ICU serum albumin levels in patients with AP demonstrated a significant non-linear negative association with in-hospital mortality. An ALB level of 28 g/L was identified as an important clinical inflection point for distinguishing high-risk patients. Early monitoring and attention to ALB levels are crucial for risk stratification and clinical decision-making in AP patients.