Background: Men are at increased risk of dying by suicide but are less likely to seek help and to be identified as at-risk. Relatives are therefore important gatekeepers in suicide prevention for men, but often lack the knowledge regarding warning signs, communication, and the referral for help, and are usually highly burdened themselves. Aims: In this study, an online program has been developed for relatives of men with suicidal ideation. Efficacy of the intervention was evaluated at post-measurement and follow-up. Methods: A total of 121 individuals was enrolled in the study and randomly assigned to either a treatment or waitlist control group. The intervention consisted of four modules, including psychoeducational content, short films with affected men and experts, and audio plays with fictional dialogues between men and their relatives. Primary outcomes were knowledge and competence; secondary outcomes were depression (PHQ-9), stress (PSS), and burden due to the support of the male relative (BAS, IEQ). Results: There was a significant increase in (perceived) knowledge, self-efficacy, and perceived preparedness in the intervention group compared to the waitlist control group. There also was a significant decrease in depressive symptoms in the intervention group compared to the waitlist control group. No significant decrease in distress and reluctance to intervene could be found. Conclusions: A short online programme can prepare relatives for interacting with men with suicidal ideation and lead to an increase in knowledge and self-efficacy. It can also support relatives by providing them with knowledge and strategies.