Improving teachers’ knowledge and skills in dealing with adolescent suicidality may be important for prevention. We evaluated the feasibility and acceptability of a theatre-based gatekeeper teacher training for adolescent suicide prevention (SPES project). Self-reported changes in knowledge and self-efficacy were also investigated. Based on a quasi-experimental repeated measures design, secondary schools were invited to participate in one of two intervention groups (theatre workshop or attending a theatre performance) or a control group (no intervention). Assessments were conducted prior to the intervention, shortly following it and 3 months later. Attendance and retention were indicators of feasibility. Knowledge and self-efficacy were assessed longitudinally using anonymous questionnaires. Data were analyzed using linear mixed models. 191 teachers (84.3% women; Mage = 46.8, SD = 9.8) participated in the study (63 workshop, 66 performance and 62 control group). Attendance was 92% in the workshop group and 94% in the performance group. The retention rate after three months was 51% in the workshop group and 53% in the performance group. Teachers in both groups reported greater knowledge of adolescent suicide (p < 0.001) and higher gatekeeper self-efficacy (p < 0.05) than the control group. The SPES project was found to be feasible and acceptable. Both the workshop and the performance viewing may improve teachers’ knowledge and self-efficacy in recognizing signs of suicide risk in adolescents.