Objective: Playground injuries result in over 200,000 US pediatric emergency department visits annually. One strategy to reduce injuries is improved adult supervision. The Stamp-in-Safety programme, which involves supervisors stamping rewards for children playing safely, has been demonstrated in preliminary classroom-based work to reduce child risk-taking and increase the quality of adult supervision. This study evaluated the efficacy of delivering Stamp-in-Safety to playground supervisors via the Internet. Design: Pre-post design following a pragmatic trials approach. Setting: Twenty-four preschools in the USA. Method: Guidance for implementing the Stamp-in-Safety programme was delivered to preschool supervisors via the Internet. Supervisors and preschool directors completed online surveys concerning self-efficacy to keep children safe, playground supervision behaviour, perceptions about playground safety, and knowledge about playground supervision/Stamp-in-Safety both prior to and after the intervention was implemented. They completed a usability survey after using the online intervention. Results: Ratings of usability and satisfaction with the programme were high, indicating feasibility. Paired-samples t-tests showed some improvement in supervisors' self-efficacy, and improvement in most aspects of supervisory behaviours, perception of playground safety, and knowledge about playground supervision/ Stamp-in-Safety. Conclusion: Results suggest the Stamp-in-Safety programme can be delivered successfully via Internet. A public health challenge is translating effective interventions to broad dissemination. Internet delivery of Stamp-in-Safety holds the promise to overcome this challenge and improve playground supervision practices.