Introduction Overweight or obesity might be protective factors of tuberculosis (TB), but the evidence is inconclusive. The objective of study was to evaluate association between BMI and incident TB. Methods The National Health Insurance database was used. Eligible participants were individuals aged 20–89 years without history of TB before 2007, and who underwent national health examinations between January 2002 and December 2006. The latest record of BMI was used as the exposure and categorized as follows: <18.5, 18.5–23, 23–25, 25–30, and 30 kg/m2. TB was defined as the first recorded diagnosis of TB, using ICD-10 between January 2007 and December 2013. Results Among 301,081 individuals, 3,772 (1.26%) incident TB cases were detected. The incidence rate of the event was 19.65 per 10,000 person-years. After adjusting age, sex, household income, smoking status, alcohol use, and diabetes, incident TB was decreased as BMI was increased in an inverse dose-response relationship. However, when stratified by age and sex, BMI >30 kg/m2 did not show protective effect of TB in female under 50 years. Additionally, BMI >30 kg/m2 did not decrease incident TB in diabetics. Conclusion Our study suggests that high BMI might be associated with decreased risk of TB. However, very high BMI did not reduce the risk of TB in young females or diabetics participants with in Korean population.