Aim This work describes an extensive systematic literature review on assessment instruments for dementia in persons with intellectual disability (ID). Existing instruments for the detection of dementia in persons with ID were collected and described systematically. This allows a direct and quick overview of available tools. Additionally, it contributes to the availability and usability of information about these instruments, thus enhancing further developments in this field. Methods A systematic literature search in five databases (CINAHL, PsycInfo, PubMed, Scopus, and Web of Science) was conducted. In order to include gray literature an invisible college approach was used. Relevant studies were identified and selected using defined inclusion and exclusion criteria. After the selection process all instruments were coded and classified. It was determined which concepts they assess, whether they were especially developed or adapted for persons with ID, and whether they were designed to assess dementia. The selection of relevant papers, as well as the coding of instruments was done independently by two researchers. Results In total, 97 records met the search criteria. Out of these, 114 different instruments were extracted. There were 79 instruments to be completed by the person with ID, and 35 informant-based instruments. Additionally, four test batteries were found. Some of these instruments were neither designed for the assessment of dementia, nor for persons with ID. Conclusions There are a variety of different tools used for the assessment of dementia in ID. Nevertheless, an agreed-upon approach or instrument is missing. Establishing this would improve the quality of assessment in clinical practice, and benefit research. Data collected would become comparable and combinable, and allow research to have more informative value.