Background <p>Evidence linking insomnia to suicidal risk in depressed youth is largely cross-sectional, limiting inference about temporal direction. We examined whether baseline insomnia severity predicts subsequent suicide attempts in this population.</p> Methods <p>We conducted a 6-month prospective cohort study of depressed youth recruited from a psychiatric hospital from January 2022 to December 2023. We examined the association between baseline insomnia severity and time to first suicide attempt using Kaplan–Meier curves and multivariable Cox proportional hazards models.</p> Results <p>A total of 561 participants were included in the final analysis (428 female participants [76.3%]). At baseline, 458 participants met the criteria for insomnia (Athens Insomnia Scale [AIS] ≥ 6; 81.6%). In multivariable Cox models, insomnia severity was associated with a small but statistically significant higher hazard of suicide attempts (HR = 1.07, 95% CI 1.01–1.13, <i>p</i> = 0.038). Kaplan–Meier curves also showed a significant difference in time to first suicide attempt between the insomnia and non-insomnia groups (<i>p</i> &lt; 0.001). In addition, greater depressive severity (HR = 1.11, 95% CI 1.04–1.18, <i>p</i> = 0.002), and a lifetime history of suicidality (HR = 2.14, 95%CI 1.31–3.48, <i>p</i> = 0.004) predicted follow-up attempts, whereas a longer illness duration (&gt;24 months) was associated with a lower hazard (HR = 0.34, 95% CI 0.17–0.69, <i>p</i> = 0.005).</p> Conclusions <p>Greater baseline insomnia severity was associated with an increased risk of subsequent suicide attempts in youth with depression. The findings support early identification and intervention for suicidal risk in this vulnerable population.</p>