Background and Aim: Scarce data exist on the true incidence of postoperative atrial fibrillation (POAF) after acute abdominal surgery and associated outcomes. The current study aimed to identify the frequencies of clinically recognized POAF and associated complications, along with their risk factors. Methods: This study was a prospective, single-center cohort study of unselected adult patients referred for acute abdominal surgery during a 3-month period. Through careful review, demographics, comorbidity, and surgical characteristics were prospectively drawn from medical charts. The primary outcome was clinically recognized POAF occurring in-hospital. Logistic regression was used to determine the risk factors of POAF and associated complications. A subgroup was enrolled in a feasibility study of peri- and postoperative continuous cardiac rhythm monitoring. Results: In total, 450 patients were enrolled. Clinically recognized in-hospital POAF was observed in 22 patients (4.9%). All cases were observed in patients aged ≥60 years, corresponding to 22 of 164 patients (13.4%). Multiple risk factors were observed, such as age, prior atrial fibrillation, heart failure, hypertension, diabetes mellitus, chronic renal disease, and major (vs. minor) surgery. POAF was associated with severe in-hospital complications (POAF group 45.5% vs. non-POAF group 8.6%, p <.001) and in-hospital mortality (POAF group 13.6% vs. non-POAF group 3.0%, p =.043). In total, 295 patients were monitored by continuous cardiac rhythm monitoring for 12,148 h, yielding five patients with asymptomatic AF. Conclusions: In conclusion, this prospective study of POAF in patients undergoing acute abdominal surgery showed that one in 20 patients developed clinically recognized in-hospital POAF. Multiple risk factors of POAF were identified. POAF was associated with severe complications up to 30 days after surgery.