Introduction <p>This study aimed to investigate the relationship between BMI and total lumbar BMD in obese adults.</p> Methods <p>The current cross-sectional study included 3,708 obese individuals. The data on BMI, total lumbar BMD, and other covariates were obtained from the National Health and Nutritional Examination Survey (NHANES) (<a href="http://www.cdc.gov/nchs/nhanes/">http://www.cdc.gov/nchs/nhanes/</a>) between 2011 and March 2020 prior to the COVID-19 pandemic. Multivariate logistic regression models were utilized to investigate the association between BMI and total lumbar BMD. Smooth curve fittings and generalized additive models were used to analyze the potential non-linearity.</p> Results <p>A total of 3,708 participants (1,610 males and 2,098 females) were included in the study. In multivariate regression analysis, the association between BMI and total lumbar BMD was positive (β = 0.003, 95% CI: 0.002, 0.004). This relationship still existed after being adjusted for gender, age, and race (β = 0.003, 95% CI: 0.002, 0.004) and fully adjusted for all covariates (β = 0.003, 95% CI: 0.002, 0.004). In threshold effect analysis, the relationship between BMI and total lumbar BMD followed a U-shaped curve, with the inflection point at 36.1&#xa0;kg/m².</p> Conclusion <p>The present study revealed a positive association between BMI and lumbar BMD in obese adults, the association notably following a U-shaped curve with an inflection point at a BMI of 36.1&#xa0;kg/m².</p>