<p>Gatekeeper training enables teachers to identify and refer students at risk of suicide and can play a role in school-based prevention efforts. A cluster-randomized controlled trial in Chinese schools assessed the impact of the Life Gatekeeper Training Program (LGTP), a standardized one-day programme comprising eight sessions, delivered through a train-the-trainer cascade. School-based clusters were randomized 1:1 to either the LGTP intervention group (42 schools, <i>n</i> = 2,051; female, 1,536 (74.9%); male, 515 (25.1%)) or a waitlist control group (42 schools, <i>n</i> = 2,089; female, 1,556 (74.5%); male, 533 (25.5%)). Primary outcomes were stigma against suicide, suicide literacy, perceived competence and willingness to intervene. Assessments were conducted at baseline, after training and at 6-month and 12-month follow-ups with a 91.4% retention at 12 months. Intention-to-treat analyses using linear mixed models showed significant improvements in all primary outcomes for the intervention group after training: stigma (<i>β</i> = –0.80, 95% confidence interval (CI) (–0.87, –0.73)), suicide literacy (<i>β</i> = 0.98, 95% CI (0.92, 1.05)), perceived competence (<i>β</i> = 1.03, 95% CI (0.97, 1.10)) and willingness to intervene (<i>β</i> = 0.76, 95% CI (0.70, 0.82)); all false discovery rate-adjusted <i>P</i> values &lt; 2.0 × 10⁻¹⁶. Effects remained significant at 6 and 12 months, with reduced but sustained effect sizes. No adverse or serious adverse events were reported. This trial supports LGTP as a brief, well-tolerated and effective intervention for enhancing suicide prevention competencies in schools. Chinese Clinical Trial Registry: <a href="https://www.chictr.org.cn/showproj.html?proj=186487">ChiCTR2200066142</a>.</p>