Environmental pollutants may play a role in the aetiology of obesity beyond conventional factors. The associations between environmental exposure to aldehydes and obesity remain unclear. The objective of this study is to determine whether aldehyde exposure is associated with obesity in adults. We analysed data from 1977 participants in the National Health and Nutrition Examination Survey (NHANES) 2013-2014 aged ≥ 18 years. Obesity was assessed through body mass index (BMI) measurements. Generalized linear regression and restricted cubic spline models were analysed to assess the association between aldehydes and outcomes. After multivariable adjustment, isopentanaldehyde was inversely associated with obesity, while no significant association was observed between any other aldehydes and obesity. Compared with the lowest quartile, the adjusted odds ratio (OR) of obesity with a 95% confidence interval (CI) for the highest quartile was 0.50 (0.35, 0.70) for isopentanaldehyde. Analyses using a restricted cubic spline indicated that the association between isopentanaldehyde and obesity is nonlinear. Threshold effect analysis demonstrated that the inflection point of isopentanaldehyde was 1.26 ng/ml. Each 1-fold increase in isopentanaldehyde exhibited an 18% decrease in the odds of obesity (OR 0.82, 95% CI 0.79-1.09) on the left side of the inflection point and an 81% decrease (OR 0.19, 95% CI 0.08-0.45) on the right side of the inflection point. Similar associations were also observed among isopentanaldehyde and abdominal obesity, BMI, and waist circumference. These cross-sectional results show a nonlinear and inverse association between isopentanaldehyde and obesity.Copyright © 2020 Elsevier Inc. All rights reserved.