The association between body mass index (BMI) in young adulthood and long-term risk of atrial fibrillation (AF) has not yet been examined for men. We conducted a population-based 36-year cohort study to examine the BMI-associated risk of AF in 12,850 young men who had BMI measured at their examination of fitness for military service. AF was identified from the Danish National Registry of Patients, covering all Danish hospitals since 1977. We began follow-up on the twenty-second birthday of each subject and continued until the occurrence of AF, emigration, death, or December 31, 2012. We used Cox regression to compute hazard ratios (HRs) with 95% confidence intervals (CIs), adjusting for education and height. The cohort contributed a total of 375,888 person-years of follow-up and the median follow-up time was 26 years (mean 29 years). The incidence of AF per 100,000 person-years was 53 for men of normal weight (BMI: 18.5 to 24.9 kg/m²), 54 for underweight men (BMI <18.5 kg/m²), 106 for overweight men (BMI: 25.0 to 24.9 kg/m²), and 144 for obese men (BMI ≥30 kg/m²). With normal weight as the reference group, the adjusted HR for AF was 0.99 (95% CI 0.52 to 1.87) for underweight men, 2.08 (95% CI 1.48 to 2.92) for overweight men, and 2.87 (95% CI 1.46 to 5.62) for obese men. The adjusted HR associating 1 unit increase in BMI with AF was 1.12 (95% CI 1.07 to 1.16). In conclusion, overweight and obese young men had more than twice the risk of AF compared with young men of normal weight.