BackgroundSoutheast Asia has the highest suicide mortality worldwide. To improve our knowledge on the effectiveness of interventions for suicidal ideation (SI) in individuals with depression in Indonesia, we conducted a secondary analysis of a randomised controlled trial.ObjectiveWe explored whether an internet-based behavioural activation (BA) intervention (‘Guided Act and Feel Indonesia’ (GAF-ID)) was superior in targeting SI compared with online-delivered psychoeducation (PE).MethodsIn total, 313 participants were randomised between treatment allocation. The SI item of the Patient Health Questionnaire-9 was the primary outcome measure. Mediation analyses were conducted to identify if BA at week 10 mediated the relationship between intervention and SI at week 24.FindingsThe GAF-ID intervention was not superior in reducing SI compared with online minimal PE at week 10 (OR 0.61, 95% CI (0.37 to 1.01)), nor at week 24 (OR 0.84, 95% CI (0.47 to 1.52)). SI at week 24 was not mediated by BA at week 10 (b=−0.03, 95% CI (−0.05 to 0.00), p=0.07).ConclusionsIn individuals with depression in Indonesia, the GAF-ID intervention was not superior in reducing self-reported SI compared with PE. Also, the association between treatment condition and SI at week 24 was not mediated via BA at week 10.Clinical implicationsThis study supports the need for further research on the efficacy of psychological treatments targeting SI in the Southeast Asia context.