Background: Instrumental activities of daily living (IADL) are complex activities which involve multiple cognitive processes, and which are expected to be susceptible to the early effects of cognitive impairment. Informant-based questionnaires are the most common tools used to assess IADL performance in dementia, but must be adjusted for use in early stages of impairment. Objective: To investigate the differences in IADL on the continuum of cognitive decline (i.e. no cognitive decline - subjective cognitive decline - mild cognitive impairment- mild dementia - moderate dementia) using the Spanish version of the Amsterdam IADL Questionnaire (A-IADL-Q). Methods: A total of 500 volunteer participants were included: 88 participants with no signs of cognitive decline, 109 participants with subjective cognitive complaints, 114 participants with mild cognitive impairment (MCI), 81 participants with mild dementia and 108 participants with moderate dementia. IADL was assessed with the A-IADL-Q, a computerized and adaptive questionnaire that calculates scores according to the specific pattern of responses of each participant. The data were examined by ANOVAs and regression analysis. Multinomial logistic regression analysis was used to evaluate the capacity of the A-IADL-Q to distinguish between diagnostic groups. Results: Participants with no cognitive decline and those with subjective cognitive decline obtained higher A-IADL-Q scores than MCI participants, and participants with MCI obtained higher scores than patients with dementia. The A-IADL-Q showed excellent discrimination between non-cognitive impairment and dementia, and significant but low discrimination between non-cognitive impairment and MCI. Conclusion: A-IADL-Q can discriminate IADL functioning between groups across the dementia spectrum.