INTRODUCTION: Atrial fibrillation (AF) is characterized by high frequency stimulation of the atrium, causes dyssynchronous atrial contraction and irregular ventricular excitation. It is the most known cardiac arrhythmia in adults, doubles the risk of stroke five times and is associated with an increasing public health burden. This study was aimed to assess the magnitude and associated factors of atrial fibrillation and its complication among adult rheumatic heart diseases patients in governmental hospitals in Bahir Dar town, Northwest Ethiopia 2024. METHODS: An institutional based cross-sectional study design was conducted with a sample size of 421. A simple random sampling technique was used to select participants. The data were entered into the Statistical Package for the Social Sciences (SPSS) version 26 for analysis. Adjusted Odds Ratio (AOR) with a 95% confidence interval was used to determine associated factors of atrial fibrillation. A binary logistic regression model was used, and a P-value <
0.05 in multivariate was considered as a statistically significant. RESULTS: The response rate was 95% and atrial fibrillation was developed in 51.2% of patient. Majority of them were Female (56%). The median age of patients was 41, with an interquartile range (IQR) of 26-51. Age >
50 years old (AOR = 7.20(2.03-25.46)), sever tricuspid regurgitation 4.50(1.18-17.20)), and left ventricular ejection fraction (LVEF) % (AOR = 0.94(0.89-0.99)), left atrium size (AOR = 1.23(1.14-1.33)) were independently associated with atrial fibrillation. For every unit increment of left ventricular ejection fraction in percent, the odds of developing atrial fibrillation decreased by 6%. For every unit increment of left atrial size in millimeter CONCLUSION: More than half of the study participants were found to have atrial fibrillation in patients with rheumatic heart disease. Being age >
50 years old, left atrium size, severity of tricuspid regurgitation (severe), and LVEF% were associated in developing atrial fibrillation. The atrial fibrillation was linked to an increased risk of ischemic stroke, heart failure, systemic thromboembolism, and death.