Background: The nature of stigmatizing attitudes towards children and adolescents with mental health problems has received little empirical attention, despite consensus that such attitudes are widespread. As a consequence, much less is known about stigma in childhood and adolescence and methods of stigma measurement are frequently borrowed from the adult literature. For research on this topic to develop, a theoretically based and developmentally appropriate measure is needed. This study aimed to develop a theory-based peer stigma questionnaire suitable for children and adolescents. Method: Participants were 562 children and adolescents aged 9-16 years (M = 12.99 years; SD = 1.6 years) in the Republic of Ireland, 316 female, all were White. The Peer Mental Health Stigmatization Scale (PMHSS) contains 24 statements (negative and positive) about peers with mental health problems that are rated on a 5-point scale. Participants also completed the Strengths and Difficulties Questionnaire. Re-test data was collected after 2 weeks from 109 participants. Results: Principal Components Analysis on the negative statements indicate the presence of two components: Stigma Agreement, personal endorsement of stigmatising statements and Stigma Awareness: awareness of prevailing societal stigma towards youth with mental health problems. The positive statements include three components: Intellectual Ability, Recovery and Friendship. Conclusions: The PMHSS is a psychometrically sound instrument with good retest reliability suitable for use with older children and teenagers. Initial use of the scale suggests that personal endorsement of stigma is lower than perceptions of public stigma.