Introduction: Crisis intervention aims to prevent hospital admissions by providing rapid assessment and intensive support in the community. Interest is growing in the potential of crisis care for people with intellectual disabilities who often have co-occurring psychiatric disorders and challenging behaviors. They are at high risk for admission to psychiatric hospitals when they experience acute mental health crises. This review reports on a systematic search and synthesis of the available evidence on the effectiveness of crisis care in reducing psychiatric admission in adults with intellectual disabilities and mental illness and/or challenging behavior. Methods: An electronic database search on EMBASE, MEDLINE, PsycINFO, CINAHL Plus, and Web of Science databases was conducted from inception until July 2021 initially, and an updated search with the same search terms was carried out until December 2022 to identify any articles that examined crisis interventions for adults with intellectual disabilities and mental illness and/or challenging behavior. All included studies were assessed for methodological quality and results were integrated through narrative synthesis. Results: A total of nine studies met the inclusion criteria, including one randomized controlled trial and eight cohort single-group pre-post studies. Study quality was deemed to be weak. Based on the available evidence, crisis interventions may hold potential for preventing psychiatric admission among adults with intellectual disabilities who have mental illness and/or challenging behavior, as well as for improving outcomes related to psychological and social functioning, service satisfaction, and cost-effectiveness. Conclusion: There is some evidence that crisis interventions can contribute to the reduction of psychiatric admission among adults with intellectual disabilities and mental illness and/or challenging behavior. However, definitive conclusions could not be drawn due to low certainty of evidence presented in the current research on the topic. Further studies should focus on the essential components involved in crisis care including service models to provide evidence for clinical and cost-effectiveness that can lead to optimization of care delivery.