Obesity has been linked to atrial fibrillation (AF) burden and severity, and epidemiological studies suggest that AF is more prevalent in whites than Asian. We aimed to investigate whether obesity mediates associations with AF in Europe and Asia using patient-level data comparisons of two cohort studies. Using Korean National Health Insurance Service's Health Screening (NHIS-HealS) and U.K. Biobank data, we included 401,206 Korean and 477,926 British aged 40-70 years without previous AF who received check-ups. The incidence and risk of AF were evaluated regarding different body mass index (BMI) values. The obese proportion (BMI ≥ 30.0 kg/m, 2.8% vs. 24.3%, P < 0.001) was higher in the U.K. than the Korean. In the Korean and U.K. cohort, the age- and sex-adjusted incidence rates of AF were 4.97 and 6.54 per 1000 person-years among obese individuals. Compared to Koreans, the risk of AF was higher in the British population, with adjusted hazard ratios of 1.41 (Korea, 95% CI 1.26-1.58) and 1.68 (UK, 95% CI 1.54-1.82) in obese participants (P for interaction < 0.05). Obesity was associated with AF in both populations. British subjects had a greater incidence of AF related to the high proportion of obese individuals, especially participants in the obesity category, the risk of AF also increased.