Abstract Purpose The study of the relationship between body weight and health in old age has attracted increasing interest. The aim of the present study is to investigate the association of body mass index (BMI) with functional and cognitive status in a group of nonagenarians. Methods We analyzed 475 participants (348 women, 127 men; median age 92 years) from the Mugello study. Participants were evaluated through laboratory, instrumental examinations and questionnaires. Results By grouping the participants according to BMI categories, a better perception of health and nutritional status and a lower prevalence of sarcopenia (p < 0.05) were observed in participants with overweight and obesity compared to participants with normal weight or underweight. Concerning functional and cognitive measures, overweight and obese participants showed significantly worse performance on short physical performance battery and timed up and go tests and better performance on the mini-mental state examination (MMSE). As regards the other tests performed, no statistically significant differences were observed. In a multivariate logistic regression analysis adjusted for possible confounding factors, participants with BMI ≥ 30 kg/m² showed lower probability to achieve poor performance on the MMSE (OR 0.42; 95% CI 0.19–0.94; p = 0.035). Conclusion Our results support the hypothesis that in nonagenarians, a higher BMI is associated with better cognitive ability. Further studies are needed to explore the mechanisms underlying this association.