Background: Hospital-treated self-poisoning is common, with limited effective interventions for reducing subsequent suicidal behaviour. Aims: To test the efficacy of a postcard intervention to reduce suicidal behaviour. Method: Randomised controlled trial of individuals who self-poisoned (n = 2300), the intervention consisted of nine postcards sent over 12 months versus usual treatment. Outcomes assessed at 12 months (n = 2113) were suicidal ideation, suicide attempts and self-cutting (proportion and event rates). Results: There was a significant reduction in any suicidal ideation (relative risk reduction (RRR) = 0.31, 95% CI 0.22–0.38), any suicide attempt (RRR = 0.42, 95% CI 0.11–0.63) and number of attempts (incidence rate ratios (IRR) = 0.64, 95% CI 0.42–0.97). There was no significant reduction in any self-cutting (RRR = 0.14, 95% CI –0.29 to 0.42) or self-cutting events (IRR = 1.03 95% CI 0.76–1.39). Conclusions: A postcard intervention reduced suicidal ideation and suicide attempts in a non-Western population. Sustained, brief contact by mail may reduce suicidal ideation and suicide attempts in individuals who self-poison.