Sexual and gender minority youths are more likely to consider, attempt, and die by suicide than are heterosexual and cisgender youths, yet little is known about how to predict future attempts or transitions from suicidal thoughts to behaviors. Additionally, adaptive measurement of psychopathology is a promising approach that may help characterize risk in this population. This study examined the validity of the Computerized Adaptive Test for Suicide Scale (CAT-SS) in predicting suicide attempts and the transition from suicidal ideation to attempt. The CAT-SS was administered to participants of two ongoing cohort studies of sexual and gender minority adolescents and young adults (N=1,006). Survival analyses examined longitudinal associations between CAT-SS scores and time to suicide attempt. Analyses were conducted for the full sample and stratified by those with and without a history of suicidal ideation, with comparisons between adaptive and static measures of depressive symptoms. The CAT-SS predicted future suicide attempts in the overall sample (hazard ratio [HR]=1.34, 95% confidence interval [CI]=1.03-1.74). Among youths without a history of suicidal ideation, social support reduced the risk for attempts (HR=0.66, 95% CI=0.45-0.96). Among youths with a history of ideation, predictors of the transition from suicidal thoughts to attempts included baseline CAT-SS score (HR=1.51, 95% CI=1.06-2.15) and victimization (HR=2.48, 95% CI=1.10-5.59). Risk and protective factors for suicide attempts differed between youths with and without a history of suicidal thoughts. The CAT-SS had validity in predicting future risk of the sample overall and of youths with suicidal ideation.