The aim of this study was to determine reliability and validity of the Spanish version of the Disability Assessment for Dementia Scale (DAD-E) in the following areas: content, response process, internal structure, and relation to other constructs.We designed a cross-sectional observational study. The DAD-E was administered to 132 participants diagnosed with mild cognitive decline, prodromal Alzheimer disease, Alzheimer disease, or no cognitive decline. For the reliability study, we performed analyses of internal consistency, test-retest, and equivalent measures. To study validity, we performed item analysis, principal components analysis, and correlations with other measures.The sample was composed of 37 healthy participants (28%) and 95 patients (72%). In the total scale, Cronbach alpha was 0.963, intraclass correlation coefficient in the test-retest analysis was 0.983 (95% CI [95% confidence interval] = 0.969-0.991), and the analysis for equivalent measures was 0.949 (95% CI = 0.897-0.975). Out of the 40 items, we found that 37 presented a correlation index with the total score above 0.40. The principal components analysis suggests that 61.7% of the variance is explained by a single component that groups all scores on Activities of Daily Living. The DAD total score presents correlations with Barthel's Index of 0.882 (P = 0.000) and with Lawton and Brodie's Index of 0.877 (P = 0.000) and with the Mini Mental State Examination of 0.679 (P = 0.000).The DAD-E is a reliable and valid instrument to assess functional disability in people with cognitive decline in Spanish population.