Objective <p>Depression affects approximately 280 million individuals&#xa0;globally, with suicidal ideation occurring in 37.7% of patients with&#xa0;major depressive disorder. Cognitive behavioral therapy (CBT) has&#xa0;demonstrated efficacy in reducing suicidal thoughts and behaviors;&#xa0;however, evidence for online delivery remains limited and inconsistent,&#xa0;with no studies examining effects on cognitive flexibility in individuals&#xa0;with recent suicidal behavior. This randomized controlled trial examined&#xa0;the efficacy of a 20-session online CBT (OCBT) program on suicidal&#xa0;ideation, cognitive flexibility, and hopelessness in individuals with&#xa0;depression who had attempted suicide.</p> Methods <p>This was a randomized controlled trial with four assessment points (baseline, 1-month, 3-month, and 6-month follow-ups). Fifty individuals with depression who had attempted suicide at least once in the previous six months were recruited from the psychiatry outpatient clinic of a state hospital between June 2021 and November 2022. Participants were randomly assigned to an intervention group (n = 25) receiving 20 sessions of individual OCBT or a control group (n = 25) receiving standard psychiatric treatment only. Both groups continued pharmacological treatment. Data were analyzed using repeated-measures ANOVA.</p> Results <p>The intervention group demonstrated significantly lower suicidal ideation (F = 72.589;<i> p </i>&lt; .001; ηp² = 0.602), greater cognitive flexibility (F = 223.568; <i>p</i> &lt; .001; ηp² = 0.823), and lower hopelessness (F = 19.688;<i> p</i>  &lt; .001; ηp² = 0.293) compared with controls. Significant group × time interactions confirmed differential trajectories for all primary outcomes (all<i> p</i> &lt; .001). However, the effect on suicidal ideation attenuated by the 6-month follow-up (<i>p</i> = .067), whereas improvements in cognitive flexibility and hopelessness were sustained. Depression and anxiety scores also decreased significantly in the intervention group at 3&#xa0;months (<i>p</i> = .015 and<i> p</i> = .002, respectively).</p> Conclusions <p>OCBT significantly reduced suicidal ideation and hopelessness while enhancing cognitive flexibility in individuals with depression and recent suicide attempts. To our knowledge, this is the first RCT to demonstrate that online CBT can improve cognitive flexibility in this population. Effects on suicidal ideation diminished by 6&#xa0;months, suggesting the need for maintenance sessions. OCBT is a feasible and effective adjunctive intervention for this high-risk population, which may reduce treatment barriers related to geography and stigma.</p>