Background: Apathy is a prominent demotivational symptom in neurodegenerative diseases, such as Amyotrophic lateral sclerosis (ALS) and Alzheimer’s disease (AD). It is considered a multidimensional syndrome, composed of different subtypes which can be measured using the Dimensional Apathy Scale (DAS), a 24 item tool assessing Executive, Emotional and Initiation apathy, independent of motor disability. Due to increasing diagnosis, awareness and impact of apathy, a concise yet comprehensive measure is needed in clinic. The aim was to reduce the DAS using a large mixed neurodegenerative disease sample (AD and ALS) to form the brief DAS (b-DAS). Method: Data from 102 non-demented ALS patients and 102 AD patients of responses to the informant/carer DAS were utilised, with additional availability of informant/carer Apathy Evaluation Scale (AES) and the Geriatric Depression Scale-Short form (GDS-15), standard apathy and depression measures. Mokken analysis was performed on each DAS subscale (Executive, Emotional and Initiation) for initial item reduction based on discrimination (Hi). Item endorsement (mean item score) was also examined. Item-total correlational analysis was performed, with the AES total, to determine convergent validity, and the GDS-15 total, to determine divergent validity, to establish the final structure of the b-DAS. Results: AD and ALS patients were well matched for years of education, but differed on gender distribution and age. However, there was no correlation with age and no gender differences on apathy or depression. Mokken analysis on each DAS subscale resulted in all 8 Executive and all 8 Initiation items being retained, with 3 Emotional items showing a weak Hi (< .3) and were consequently removed. Of the remaining items, those with a stronger positive correlation with the AES (r > .5) and also a moderate to weaker correlation with the GDS-15 (r < .5), as well as those theoretically coherent to each subscale, were selected. This resulted in the b-DAS composed of 9 items, equally weighted over the Executive, Emotional and Initiation subscales. Conclusion: The b-DAS is a robust yet short multidimensional apathy instrument composed of 9 items that correlate with a gold-standard apathy measure, whilst reducing the association with depression. It is appropriate for use in the clinic and research to quickly and comprehensively screen for apathy subtype impairments in neurodegenerative disease.