Globally, 1–3% of the population has an intellectual disability, but some remain undiagnosed, resulting in limited access to essential health and social care services, poor health outcomes, and higher risks of homelessness, substance abuse, and imprisonment. A formal diagnosis enables early intervention and support. A scoping review was conducted to explore research on undiagnosed intellectual disability, screening processes, and identifying indicators. Method: The scoping review search was conducted using academic databases such as CINAHL, PsycINFO, Scopus, and PubMed, along with eight grey literature sources. In addition, the reference lists of the included studies were explored. Information specialists supported and guided the search process. The search included qualitative, quantitative, review, and mixed-method research studies published in English between 2000 and 2024. Two reviewers screened papers for eligibility by title, abstract, and full text. Result: A total of 11,475 papers were screened, with 57 papers from various countries included in the review. Indicators of intellectual disability were identified in three settings: (1) educational settings (preschool, primary, and secondary schools); (2) social care services, including homeless and community services; and (3) criminal services, such as courts, probation, and prisons, highlighting the wide applicability of findings. These indicators were linked to “experiences”, “behaviours”, “challenges”, and “observations”. Conclusion: This review emphasised the importance of early diagnosis by adopting appropriate assessment tools, which require national screening policies. It also highlighted the essential role of professionals working in intellectual disability services in identifying and supporting undiagnosed individuals, particularly within high-risk populations. Implication: The review’s findings will guide policy, practice, and research recommendations for enhancing the early identification of people with intellectual disabilities.