Suicide is a leading cause of death among young people, yet long-term studies of psychosocial risk factors for suicidal ideation (SI) in Asian youth are scarce. This study examined longitudinal associations between psychosocial factors and SI from adolescence to young adulthood. This study analyzed data from the Taiwan Youth Project, a 14-year school-based cohort of 5656 adolescents. Psychosocial variables and suicidal ideation were assessed across five waves. Multivariable generalized estimating equations with multiple imputation were used to examine associations. At baseline, the mean age was 14.36 years, with 50.64% being male. Over five waves of follow-up, the sample size decreased from 5541 to 2752, and the prevalence of suicidal ideation decreased from 15.68% to 2.80%. Conflict with parents [adjusted odds ratio (aOR) = 1.83, p < 0.001], maternal physical punishment (aOR = 1.48, p < 0.001), severed friendship (aOR = 1.67, p < 0.001), exposure to school bullying (aOR = 1.14, p = 0.049), illicit drug use (aOR = 3.34, p < 0.001), loneliness (aOR = 1.84, p < 0.001), depression (aOR = 2.75, p < 0.001), excessive worry (aOR = 1.77, p < 0.001), and difficulty falling asleep (aOR = 1.42, p < 0.001) and maintaining sleep (aOR = 1.80, p < 0.001) were associated with greater odds of SI. Several key psychosocial factors were associated with a higher risk of SI across adolescence and young adulthood. Identifying these factors may help early detection and intervention among at-risk youth.