Obesity is an important characteristic manifestation of metabolic syndrome (MetS), and body roundness index (BRI) is one of the anthropometric indicators associated with obesity. However, studies on the relationship between BRI and MetS risk are limited. We aimed to explore the relationship between baseline BRI and MetS in the USA population. Our study used data from the National Health and Nutrition Examination Survey from 1999 to 2018, ultimately enrolling and analysing 47 303 participants. Data-driven tertiles were used to categorise BRI levels, and multivariate logistic regression models were fitted to investigate the association of BRI with MetS in adults. In addition, receiver operating characteristic curve analysis was used to assess the ability of BRI to predict MetS. The distribution of BRI was different across ethnic groups with a gradual decrease in the proportion of non-Hispanic Whites and other races. In addition, BRI was significantly associated with traditional cardiovascular risk factors. Univariate regression analysis indicated BRI to be a moderate risk factor for MetS, and multivariate logistic regression analysis found that BRI remained an independent risk factor for MetS. After adjusting for confounding variables, a non-linear relationship was found between BRI levels and the prevalence of MetS. More importantly, BRI predicted MetS with the largest AUC among anthropometric measures. In summary, elevated baseline BRI levels are independently associated with the development of MetS, and baseline BRI may assist in identifying patients at risk for MetS, leading to early and optimal treatment to improve their outcomes.