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Background: Dementia in people with intellectual disabilities (IDs) is difficult to detect because of preexisting cognitive deficits. An effective screening method is required. The Dementia Screening Questionnaire for Individuals with Intellectual Disabilities (DSQIID) was developed as an observer rating tool to screen dementia in people with ID. The aim of this study was to verify the screening accuracy of the DSQIID for Japanese people with ID. Methods: Four‐hundred ninety‐three subjects with ID participated in this study. Caregivers who had observed the participants for more than 2 years scored the Japanese version of the DSQIID (DSQIID‐J) of the participants. Three doctors examined participants directly and diagnosed dementia using the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition criteria. To identify the key screening items that predict dementia, the specificities of a single and pairs of items with 100% sensitivity were evaluated relative to the dementia diagnosis. Results: Of 493 participants, 34 were people with Down syndrome (DS), and 459 were people without DS. Seventeen participants were diagnosed with dementia. The suitable cut‐off score of the DSQIID‐J was 10/11 (sensitivity 100% and specificity 96.8%) for screening dementia. The inter‐rater reliability, test–retest reliability and internal consistency of the DSQIID‐J were excellent. Regarding key items, there was no single item with 100% sensitivity, and the best two‐item combination was the pair of 'Cannot dress without help' and 'Walks slower' (sensitivity 100% and specificity 93.5%). Conclusions: We identified several important question items of the DSQIID‐J related to the diagnosis of dementia in people with ID. The DSQIID‐J is a useful screening tool for dementia in adults with ID. [ABSTRACT FROM AUTHOR]