This study aimed to estimate the nationwide prevalence of cardiometabolic diseases (CMD) among adults with underweight in the US general population. Using data from the National Health and Nutrition Examination Survey (1999–2020), we estimated the age-standardised prevalence of dyslipidemia, hypertension, diabetes, chronic kidney disease, CVD and the presence of zero or at least two CMD. Multivariable Poisson regressions were used to compare CMD prevalence between subgroups, adjusting for age, sex and race/ethnicity. Among the 855 adults with underweight included, the weighted mean age was 40·8 years, with 68·1 % being women and 70·4 % non-Hispanic White. The estimated prevalence rates were 23·4 % for dyslipidemia (95 % CI 19·4 %, 27·5 %), 15·6 % for hypertension (95 % CI 13·3 %, 17·8 %), 2·5 % for diabetes (95 % CI 1·5 %, 3·5 %), 7·9 % for chronic kidney disease (95 % CI 6·9 %, 8·8 %) and 6·1 % for CVD (95 % CI 4·3 %, 7·9 %). The prevalence of having zero and at least two CMD was 50·6 % (95 % CI 44·1 %, 57·0 %) and 12·3 % (95 % CI 8·1 %, 16·4 %), respectively. Non-Hispanic Black adults had significantly higher prevalence of diabetes (adjusted prevalence ratio, 3·35; 95 % CI 1·35, 8·30) compared with non-Hispanic White adults. In conclusion, approximately half of the underweight adults had at least one CMD, and 12·3 % had at least two CMD. Prevention and management of CMD in underweight adults are critical yet neglected public health challenges.