Background: The aim of this study was to determine the prevalence of dementia in a socioeconomically disadvantaged population of older adults living in the city of São Paulo, Brazil.Methods: A cross-sectional one-phase population-based study was carried out among all residents aged ≥ 65 in defined census sectors of an economically disadvantaged area of São Paulo. Identification of cases of dementia followed the protocol developed by the 10/66 Dementia Research Group.Results: Of 2072 individuals in the study, 105 met the criteria for a diagnosis of dementia, yielding a prevalence of 5.1%. Prevalence increased with age for both men and women after age 75 years, but was stable from 65 to 74 years. Low education and income were associated with increased risk of dementia.Conclusions: The prevalence of dementia among older adults from low socioeconomic backgrounds is high. This may be partly due to adverse socioeconomic conditions and consequent failure to compress morbidity into the latter stages of life. The increasing survival of poorer older adults with dementia living in developing countries may lead to a rapid increase in the prevalence of dementia worldwide.