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Introduction: New-onset atrial fibrillation (NOAF) is a common complication in critically ill patients, and sepsis significantly increases the risk. While short-term consequences have been studied, long-term outcomes of septic ICU patients who develop NOAF remain unclear. This study evaluated the association between NOAF and one-year all-cause mortality in a large septic ICU population. Methods: We conducted a retrospective cohort study using the MIMIC-IV database. Adult ICU patients meeting the Angus criteria for sepsis were included. Atrial fibrillation (AF) was identified via nurse documentation in electronic health records, an alternative to ECG-confirmed AF. NOAF was defined as new nurse-documented AF with no prior history. Patients were grouped into: no AF, preexisting AF, and NOAF. The primary outcome was one-year all-cause mortality. Kaplan-Meier survival curves were generated, and multivariable Cox models adjusted for age, gender, and SOFA score. Results: Of 546,028 patients in MIMIC-IV, 58,871 met Angus sepsis criteria. After applying study criteria, 22,800 patients were analyzed; 54% were male. Of these, 13,843 (60.7%) had no AF, 8,013 (35.0%) had preexisting AF, and 944 (4.3%) developed NOAF during their ICU stay. Compared to other groups, NOAF patients were significantly older, had higher SOFA scores, and longer ICU stays (p < 0.001 for all). One-year mortality was 62.7% in the NOAF group, compared to 51% in preexisting AF and 37% in no AF (p < 0.001). Kaplan-Meier analysis showed significantly lower survival in NOAF patients (log-rank p < 0.001). On multivariable analysis, NOAF was independently associated with increased one-year mortality (HR 1.37, 95% CI 1.25–1.49, p < 0.001). Age (HR 1.009, 95% CI 1.000–1.010, p = 0.02) and SOFA score (HR 1.10, 95% CI 1.09–1.109, p < 0.001) were also significant predictors. Conclusions: In this large retrospective cohort, nurse-documented NOAF was independently associated with increased one-year mortality in septic ICU patients. These findings highlight NOAF as a marker of poor long-term prognosis, supporting the need for targeted surveillance and post-ICU follow-up.