Suicidality is common among individuals at risk of psychosis. Emerging findings suggest that mental illness stigma contributes to suicidality. However, it is unclear whether stigma variables are associated with suicidality among young people at risk of psychosis. This longitudinal study assessed perceived public stigma and the cognitive appraisal of stigma as a stressor (stigma stress) as predictors of suicidal ideation among individuals at risk of psychosis over the period of one year. One hundred and seventy-two participants between 13 and 35 years of age were included who were at high or ultra-high risk of psychosis or at risk of bipolar disorder. At one-year follow-up, data were available from 73 completers. In multiple logistic regressions an increase of stigma stress (but not of perceived stigma) over one year was significantly associated with suicidal ideation at one-year follow-up, controlling for age, gender, symptoms, comorbid depression and suicidal ideation at baseline. Interventions to reduce public stigma and stigma stress could therefore improve suicide prevention among young people at risk of psychosis.