Introduction: The assessment of well-being is crucial for measuring the effects of psychosocial interventions in people with intellectual disabilities. The aim of this study was to adapt and validate a measure of integrative well-being, the Pemberton Happiness Index (PHI) to use it specifically for this population. Methods: The Pemberton Happiness Index was first tailored to individuals with intellectual disabilities (PHI-Intellectual Disabilities). Then a sample of 130 adults with mild-moderate intellectual disabilities completed this adapted version alongside other well-being and affective measures. Results: The psychometric properties of this new PHI-Intellectual Disabilities were shown to be very good (i.e., internal consistency, convergent and incremental validity). Conclusion: The study provides initial evidence of the reliability and validity of a brief measure of well-being that can be used for both applied and research purposes specifically for adults with mild-moderate intellectual disabilities.