PURPOSE: Self-identification of having a mental illness has been shown to be an important factor underpinning help-seeking behaviour and may mediate the relationship between personal stigma and mental health service use. This study validates a new scale for the self-identification of having a mental illness among a non-clinical, community cohort of young people in the UK. METHODS: Following consultation with a group of young person experts with experience of mental health problems, we evaluated the psychometric properties of the self-identification of mental illness scale (SELF-I) among 423 young people aged 13-24 years who are part of an ongoing prospective community cohort. We performed test retest reliability among a subset of 53 participants. Psychometric validation for the scale used measures of Cronbach's alpha and Pearson's correlation coefficient. Item performance was assessed along and in relation with each covariate. RESULTS: The SELF-I demonstrated robust psychometric properties including high test-retest reliability (0.95) and good internal consistency (0.87 as determined by the Cronbach's alpha). The inter-total correlations for each item, which ranged from 0.62 to 0.74, supported keeping all items in the scale. Reporting greater psychiatric symptomatology via the SDQ (β: 0.82 95% confidence interval 0.40, 1.23), psychotic-like experiences (β: 0.37 95% confidence interval 0.14, 0.59), and use of mental health services (β: 0.92 95% confidence interval 0.71, 1.13) were associated with a greater self-perception as having a mental illness (p < 0.05), providing evidence of convergent validity. As expected, we found that less intended stigmatising behaviour was associated with greater self-perceptions of having a mental illness (B: 0.18, 95% CI 0.07, 0.28). CONCLUSIONS: The SELF-I scale provides a method to gather insight into how young people, who may not identify as service users, perceive their own mental state and potential risk for developing a mental illness. This can be important for understanding perceived need for help and likelihood of using services among those with mental health problems.