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Alzheimer's disease is one of the most common causes of dementia nowadays, and its prevalence increases over time. Because of this and the difficulty of its diagnosis, accurate methods for the analysis of specific biomarkers for an early diagnosis of this disease are much needed. Recently, the levels of unfolded isoform of the multifunctional protein p53 in plasma have been proved to increase selectively in Alzheimer's Disease patients in comparison with healthy subjects, thus entering the list of biomarkers that can be used for the diagnosis of this illness. We present here the development of an electrochemical immunosensor based on nanostructured screen-printed carbon electrodes for the quantification of unfolded p53 in plasma samples. The sensor shows a suitable linear range (from 2 to 50 nM) for its application in real blood samples and a very low limit of detection (0.05 nM). The concentration of unfolded p53 has been accurately detected in plasma of elderly people in healthy conditions, subjects with mild cognitive impairment (MCI) and Alzheimer's Disease (AD) subjects, obtaining results with no significant differences to those provided by an ELISA assay. These results support the possibility of measuring unfolded p53 levels with a cheap, simple and miniaturized device with a promising future for point-of-care applications in the early diagnosis of Alzheimer's dementia. Image 1 • An immunosensor for the detection of unfolded p53 protein, an emerging biomarker for Alzheimer's Disease, was developed. • A competitive immunoassay was carried out using screen-printed carbon electrodes nanostructured with gold nanoparticles. • A high selectivity is achieved using an antibody that only reacts with an epitope present in the unfolded p53 protein. • A wide dynamic range (2–50 nM) has been obtained with excellent analytical characteristics. • This biosensor was applied for unfolded p53 quantification in plasma samples with high accuracy. [ABSTRACT FROM AUTHOR]