Four studies (total n = 961) developed and validated the Adolescent Conspiracy Beliefs Questionnaire (ACBQ). Initial items were developed in collaboration with teachers. An Exploratory Factor Analysis (Study 1, n = 208, aged 11-14) and a student focus group (N = 3, aged 11) enabled us to establish the factor structure of a 9-item scale. This was replicated via Confirmatory Factor Analysis in Study 2 (N = 178, aged 11-17), and the scale displayed good convergent (i.e., relationship with paranoia and mistrust) and discriminant validity (i.e., no relfationship with extraversion). Study 3a (N = 257) further tested convergent validity with a sample of 18-year olds (i.e., relationship with adult-validated measures of conspiracy beliefs), and demonstrated strong test re-test reliability. Study 3b (N = 318) replicated these findings with a mixed-age adult sample. The ACBQ will allow researchers to explore the psychological antecedents and consequences of conspiracy thinking in young populations.