ABSTRACTAimDespite substantial evidence in adults, data on sexual behaviour in youth with bipolar disorder (BD) are limited. We therefore examined this topic in youth with BD.MethodThe study included 625 youth (n = 246 BD, n = 233 high risk for BD [BD‐risk] and 146 healthy controls [HC]), ages 13–20 years. Semi‐structured interviews evaluated diagnoses and treatment, as well as sexual activity, risky sexual behaviour (RSB) and reasons for RSB. Analyses compared lifetime sexual activity and RSB across the three groups and evaluated correlates of sexual activity and RSB within BD and BD‐risk.ResultsLifetime sexual activity and RSB were significantly higher amongst both BD and BD‐risk youth versus HC. In univariate analyses, several overlapping clinical and suicide risk factors were associated with lifetime sexual activity amongst both BD and BD‐risk youth. In multivariate analysis amongst BD youth, comorbid substance use disorder (SUD), post‐traumatic stress disorder, and family history of anxiety were associated with lifetime sexual activity. In BD‐risk youth, older age, lifetime depression severity, comorbid SUD and interpersonal chaos were associated with lifetime sexual activity. Affective lability and family history of suicide attempt were associated with RSB.ConclusionsLifetime sexual activity was two‐fold greater, and RSB was over fifteen‐fold greater, amongst BD and BD‐risk youth versus HCs. Understanding which demographic and clinical factors are associated with lifetime sexual activity amongst BD and BD‐risk youth may enable us to identify youth more likely to engage in RSB and help us develop treatments and preventative strategies to avoid RSB accordingly.