Objective:  Evidence based on controlled studies is still limited for treatment strategies that prevent recurrence of suicide attempts. Findings from observational as well as meta‐analytic studies strongly suggest that lithium may have suicide‐protective properties. Method:  Patients with a recent suicide attempt in the context of an affective spectrum disorder (n = 167) were treated with either lithium or placebo during a 12‐month period. Results:  Survival analysis showed no significant difference of suicidal acts between lithium and placebo‐treated individuals (adjusted hazard ratio 0.517; 95% CI 0.18–1.43). However, post hoc analysis revealed that all completed suicides had occurred in the placebo group accounting for a significant difference in incidence rates (P = 0.049). Conclusion:  Results indicate that lithium treatment might be effective in reducing the risk of completed suicide in adult patients with affective disorders. Our findings contribute to the growing body of evidence suggesting a specific antisuicidal effect of lithium.