Purpose: Wellbeing refers to a person's overall happiness and satisfaction with life. Wellbeing for people with Intellectual and Developmental Disabilities (I/DD) and their families is historically significantly lower compared to the general population. It is important in the context of behavioral health treatment to not only measure the individual who is receiving treatment's overall wellbeing, but also the wellbeing of the family. The purpose of this study was to understand the initial psychometric properties of the Catalight Family Wellbeing Scale. Methods: The Catalight Family Wellbeing Scale was developed for families who have a child with I/DD. Caregivers of 3106 families who have a child with a diagnosed I/DD, including autism spectrum disorder, completed the scale as part of their onboarding for behavioral health treatment along with three other questionnaires. The psychometric properties including internal reliability and factor structure were completed as well as initial convergent and divergent validity. Results: Results of the analyses revealed very strong internal reliability and a three-factor structure. Validity analyses revealed a moderate positive relationship with parental self-efficacy and a moderate negative relationship with parental stress. Additionally, the sample population represents an ethnically diverse group with multiple co-occurring diagnosis in addition to I/DD. Conclusions: The initial psychometric properties of the Catalight Family Wellbeing Scale are positive and support the use of the scale for families who have a child with I/DD across a diverse sample.