Background: The dynamic nature of intellectual disability necessitates repeated screening. Method: The functional screening tool (FST-ID) for individuals with intellectual disabilities was carefully constructed. Results: The FST-ID General Adaptive Composite score (GAC) runs between 0 and 68 points. Criterion validity--high-positive correlations were found between FST-ID and ABAS-II (Gold standard), and between the severity of the intellectual disability and the FST-ID (r = 0.78, p < 0.001). The scale holds high-internal consistency and intra- and inter-rater reliability values. The cut-off point between mild and moderate disability is 50.4 (Sensitivity: 87%, Specificity 86%, p < 0.001) and 29.4 between moderate to severe-profound (Sensitivity: 96%, Specificity 94%, p < 0.001). Conclusions: The FST-ID presents high-psychometric properties and requires less than 5 min to complete. Using this tool may support efficient screening practices and improved supports for adults with intellectual disability.