Background There is a growing impetus across the research, policy and practice communities for children and young people to participate in decisions that affect their lives. Furthermore, there is a dearth of general instruments that measure children and young people's views on their participation in decision‐making. This paper presents the reliability and validity of the Child and Adolescent Participation in Decision‐Making Questionnaire (CAP‐DMQ) and specifically looks at a population of looked‐after children, where a lack of participation in decision‐making is an acute issue. Methods The participants were 151 looked after children and adolescents between 10–23 years of age who completed the 10 item CAP‐DMQ. Of the participants 113 were in receipt of an advocacy service that had an aim of increasing participation in decision‐making with the remaining participants not having received this service. Results The results showed that the CAP‐DMQ had good reliability (Cronbach's alpha = 0.94) and showed promising uni‐dimensional construct validity through an exploratory factor analysis. The items in the CAP‐DMQ also demonstrated good content validity by overlapping with prominent models of child and adolescent participation (Lundy 2007) and decision‐making (Halpern 2014). A regression analysis showed that age and gender were not significant predictors of CAP‐DMQ scores but receipt of advocacy was a significant predictor of scores (effect size d = 0.88), thus showing appropriate discriminant criterion validity. Conclusion Overall, the CAP‐DMQ showed good reliability and validity. Therefore, the measure has excellent promise for theoretical investigation in the area of child and adolescent participation in decision‐making and equally shows empirical promise for use as a measure in evaluating services, which have increasing the participation of children and adolescents in decision‐making as an intended outcome.