Background: Mental health (MH) problems are more common in people with intellectual disabilities (ID), yet under-diagnosis persists, which may be partly due to a lack of appropriate assessment tools. This study presents a systematic review of instruments used to assess MH problems in Spanish-speaking adults with ID. Method: Following PRISMA guidelines, a search was conducted in Web of Science, PsycINFO, and Scopus using terms related to ID, MH and assessment. From 3929 records, 12 studies met the inclusion criteria, identifying 13 instruments in Spanish. Results: The overall quality of the instruments was medium to low, with main limitations in sampling and data analysis. Most tools assessed adaptive skills, disruptive behaviour, anxiety, depression, or cognitive impairment using hetero-reported scales, mostly adapted from Anglo-Saxon instruments. Conclusion: Further research is needed to develop culturally adapted, psychometrically robust tools that allow people with ID to express their own MH needs, moving from hetero-reported approaches.