The objective of the study was to validate adapted versions of the Glasgow Anxiety Scale for people with Intellectual Disabilities (GAS-ID) simultaneously developed in English and French. A sample of 361 youth with mild to moderate intellectual disability (ID) (M = 15.78 years) from Australia (English-speaking) and Canada (French-speaking) participated in this study. The results supported the factor validity and reliability, measurement invariance (between English and French versions), a lack of differential items functioning (as a function of youth's age and ID level, but not sex in the English-Australian sample), temporal stability (over one year interval), and convergent validity (with global self-esteem and school loneliness) of a bi-factor exploratory structural equation modeling representation of the GAS-ID. The present study supports the psychometric properties of the English-Australian and French-Canadian versions of the adapted GAS-ID.