The relationship of body mass index (BMI) with lung function and COPD has been previously described in several high-income settings. However, few studies have examined this relationship in resource-limited settings where being underweight is more common. We evaluated the association between BMI and lung function outcomes across 14 diverse low- and middle-income countries. We included data from 12,396 participants aged 35–95 years and used multivariable regressions to assess the relationship between BMI with either COPD and lung function while adjusting for known risk factors. An inflection point was observed at a BMI of 19.8 kg/m 2 . Participants with BMI < 19.8 kg/m 2 had a 2.28 greater odds (95% CI 1.83–2.86) of having COPD and had a 0.21 (0.13–0.30) lower FEV 1 and 0.34 (0.27–0.41) lower FEV 1 /FVC z-score compared to those with BMI ≥ 19.8 kg/m 2 . The association with lung function remained even after excluding participants with COPD. Individuals with lower BMI were more likely to have COPD and had lower lung function compared to those in higher BMI. The association with lung function remained positive even after excluding participants with COPD, suggesting that being underweight may also play a role in having worse lung function.