Children affected by mental disorders are largely unrecognised and untreated across the world. Community resources, including the school system and teachers, are important elements in actions directed to promoting child mental health and preventing and treating mental disorders, especially in low- and middle-income countries. We developed a web-based program to educate primary school teachers on mental disorders in childhood and conducted a cluster-randomised controlled trial to test the effectiveness of the web-based program intervention in comparison with the same program based on text and video materials only and to a waiting-list control group. All nine schools of a single city in the state of São Paulo, Brazil, were randomised to the three groups, and teachers completed the educational programs during 3 weeks. Data were analysed according to complete cases and intention-to-treat approaches. In terms of gains of knowledge about mental disorders, the web-based program intervention was superior to the intervention with text and video materials, and to the waiting-list control group. In terms of beliefs and attitudes about mental disorders, the web-based program intervention group presented less stigmatised concepts than the text and video group and more non-stigmatised concepts than the waiting-list group. No differences were detected in terms of teachers' attitudes. This study demonstrated initial data on the effectiveness of a web-based program in educating schoolteachers on child mental disorders. Future studies are necessary to replicate and extend the findings.