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Background People with intellectual disability (ID) without Down syndrome (DS) are also presumed to be at higher risk of developing dementia due to their lower baseline cognitive reserve. We aimed to determine the prevalence of dementia in people with ID without DS and to identify risk factors of dementia. Methods This was a cross-sectional survey and multicenter study in Japan. Adults with ID without DS residing in the facilities were included. Caregivers in all participants were interviewed by medical specialists, and participants suspected of having cognitive decline were examined directly. DSM-5, ICD-10, and DC-LD were used to diagnose dementia. The severity of ID, educational history, and comorbidities were compared by dividing the groups into those with and without dementia. Results A total of 1831 participants were included; 118/1831 (6.44%) were diagnosed with dementia. The prevalence of dementia for each age group was 8.8%, 60–64 years; 9.0%, 65–69 years; 19.6%, 70–74 years; 19.4%, 75–79 years. Age, severity of ID, duration of education, hypertension, stroke, and traumatic brain injury were significantly associated with the presence of dementia. Conclusions Although the prevalence of dementia in people with ID without DS was found to be higher at a younger age than in the general population, the results of this study suggested that adequate education, prevention of head trauma and stroke, and treatment of hypertension may reduce the risk of dementia. These may be potentially important modifiable risk factors for the prevention of dementia in these people.