Abstract The aim of this study was to estimate the prevalence of underweight among older adults according to socio-demographic characteristics in different regions of Brazil as well as determine associated contextual and individual factors. Cross-sectional population-based study with older adults (≥ 65 years) interviewed by telephone survey in 2014. The body mass index was calculated based on weight and height. Associations were determined using Pearson’s chi-square test, considering a 5% significance level. Adjusted prevalence ratios were estimated using multilevel Poisson regression. Mean age was 73.3 years and the prevalence of underweight was 15.6% (95%CI: 14.1-17.1%). Higher prevalence rates of underweight were found among women, individuals aged ≥ 80 years, smokers and those who reported the regular consumption of beans. The prevalence rate of underweight was lower among those who reported abusive alcohol intake and those with a medical diagnosis of hypertension. The northern region of the country had the highest prevalence of underweight after adjusting for associated individual factors. The findings demonstrate the subgroups with higher prevalence rates of underweight that demand greater attention from the health services in terms of recovering of an adequate nutritional status.