Suicide among young people is an increasingly prevalent and devastating public health crisis around the world. To reduce the rate of suicide, it is important to identify factors that can help us better predict suicidal ideation and behaviors. Adolescent temperament(Effortful Control, Negative Emotionality, Positive Emotionality)may be a source of risk and resilience for the onset of suicidal ideation, plans, and attempts. The present study uses longitudinal data from a large, community sample of Mexican-origin youth (N=674),assessed annually from age 12 to 21, to examine how temperament is associated with the onset of suicidal ideation and behaviors during adolescence and young adulthood.Results indicate that higher levels of Effortful Control (Activation Control, Inhibitory Control, Attention)are associated with decreased probability of experiencing the onset of suicidal ideation, plans, and attempts, whereas higher levels of Negative Emotionality (particularly Aggression, Frustration, and Depressed Mood) are associated with increased probability of experiencing the onset of suicidal ideation and behaviors. Positive Emotionality (Surgency, Affiliation) was not associated with the onset of suicidal ideation and behaviors.Supplemental analyses showed conceptually similar findings for the Big Five, with Conscientiousness associated with decreased risk, Neuroticism associated with increased risk, and the other three dimensions showing largely null results.The findings did not vary significantly for boys and girls or for youth born in the U.S. versus Mexico. Overall, these findings suggest that adolescent temperament serves as both a protective factor(via Effortful Control/Conscientiousness) and a risk factor (via Negative Emotionality/Neuroticism) for suicidal ideation and behaviors in Mexican-origin youth.