The prevalence and impact of perioperative atrial fibrillation (AF) during an admission for major emergency abdominal surgery are sparsely examined. Therefore, this study aimed to compare the 30-day and 1-year outcomes (AF-related hospitalization, stroke, and all-cause mortality) in patients with and without perioperative AF to their major emergency abdominal surgery. All patients without a history of AF who underwent major emergency abdominal surgery from 2000 to 2019 and discharged alive were identified using Danish nationwide registries. Patients with and without perioperative AF (defined as new-onset AF during the index hospitalization) were matched 1:4 on age, gender, year of surgery, and type of surgery. The cumulative incidences and hazard ratios of outcomes were assessed using a multivariable Cox regression analysis comparing patients with and without perioperative AF. A total of 2% of patients were diagnosed with perioperative AF. The matched cohort comprised 792 and 3,168 patients with and without perioperative AF, respectively (median age 78 years [twenty-fifth to seventy-fifth percentile 70 to 83 years]; 43% men). Cumulative incidences of AF-related hospitalizations, stroke, and mortality 1 year after discharge were 30% versus 3.4%, 3.4% versus 2.7%, and 35% versus 22% in patients with and without perioperative AF, respectively. The 30-day outcomes were similarly elevated among patients with perioperative AF. Perioperative AF during an admission for major emergency abdominal surgery was associated with higher 30-day and 1-year rates of AF-related hospitalization and mortality and similar rates of stroke. These findings suggest that perioperative AF is a prognostic marker of increased morbidity and mortality in relation to major emergency abdominal surgery and warrants further investigation.