Objective: The majority of individuals with suicidal ideation do not receive help, and every year close to 800,000 people die by suicide. This study aimed to investigate the effectiveness of a guided internet-based self-help program compared to a waiting list control group in reducing suicidal ideation.

Methods: In a randomized controlled trial, 402 individuals with suicidal ideation were assigned to a guided internet-based self-help program or a waiting list control group from September 13, 2016, to September 2, 2018. The primary outcome was suicidal ideation measured with the Beck Scale for Suicide Ideation at postintervention (6 weeks after baseline).

Results: Participants assigned to the internet-based self-help program experienced at postintervention a significant reduction on the primary outcome of suicidal ideation (mean difference: 2.91; 95% CI, 1.28 to 4.54; P = .0005, Cohen's d = 0.25) compared to the waiting list control group and on the secondary outcomes of hopelessness (mean difference: 1.98; 95% CI, 0.97 to 3.99) and worrying (mean difference: 5.19; 95% CI, 2.36 to 8.10). Six months later (follow-up), the difference between the groups remained significant for suicidal ideation, hopelessness, and worrying. A total of 28 (16.8%) of the participants in the intervention group reported negative effects from the internet-based self-help program.

Conclusions: Internet-based self-help therapy was associated with a reduction in suicidal ideation at postintervention and 6-month follow-up. Some participants found it challenging to work with the therapeutic exercises, and we recommend that internet-based self-help therapy be implemented in mental health clinics or crisis lines, where support or online counseling is available.

Trial Registration: ClinicalTrials.gov identifier: NCT02872610.