Background: Despite the relevance of assessing sexual knowledge in people with Intellectual Disability, there is a lack of appropriate assessment tools to measure this domain. The current study tests the psychometric properties of the new 'Inventory of Sexual Knowledge of people with Intellectual Disability' (ISK-ID). Method: 345 individuals with mild intellectual disability completed the ISK-ID before and after the implementation of a sexual education program. Psychometric properties of the ISK-ID were analysed according to Multidimensional Item Response Theory (MIRT). Results: Its underlying factorial structure, along with parameters derived from the MIRT (item discrimination, difficulty, and participant's ability), support the use of the ISK-ID as a measure of sexual knowledge. Moreover, the ISK-ID was able to detect changes in the level of sexual knowledge resulting from educational interventions (i.e., responsiveness). Conclusions: The ISK-ID is an appropriate assessment tool to measure sexual knowledge in men and women with mild intellectual disability.