Leptin (LEP) regulates glucose metabolism and energy storage in the body. Osteoarthritis (OA) is associated with the upregulation of serum LEP. LEP promoter methylation is associated with obesity. So far, few studies have explored the association of BMI and OA with LEP methylation. We assessed the interaction between body mass index (BMI) and OA on LEP promoter methylation. Data of 1114 participants comprising 583 men and 558 women, aged 30&ndash / 70 years were retrieved from the Taiwan Biobank Database (2008&ndash / 2015). Osteoarthritis was self-reported and cases were those who reported having ever been clinically diagnosed with osteoarthritis. BMI was categorized into underweight, normal weight, overweight, and obesity. The mean LEP promoter methylation level in individuals with osteoarthritis was 0.5509 ± / 0.00437 and 0.5375 ± / 0.00101 in those without osteoarthritis. The interaction between osteoarthritis and BMI on LEP promoter methylation was significant (p-value = 0.0180). With normal BMI as the reference, the mean LEP promoter methylation level was significantly higher in obese osteoarthritic individuals (&beta / = 0.03696, p-value = 0.0187). However, there was no significant association between BMI and LEP promoter methylation in individuals without osteoarthritis, regardless of BMI. In conclusion, only obesity was significantly associated with LEP promoter methylation (higher levels) specifically in osteoarthritic patients.