First responders (FRs) are at high risk of being exposed to traumatic events in their occupational roles. Limited evidence-based support has been offered to FRs who have experienced occupational trauma using digital interventions (DIs). OBJECTIVE: To gain evidence from FRs relating to the impact of exposure to occupational trauma and describe the development of and planned protocol for testing an evidence-based, co-created DI called Sentinel to help manage and prevent trauma among FRs. METHODS: In phase 1, we conducted 52 interviews with FRs from fire and emergency, police, and emergency healthcare to explore the needs, views and experiences on using DI to support mental health. During phase 2, we developed the DI (Sentinel) content in consultation with current evidence, policy, theory and FRs experience. In phase 3, we plan to conduct a series of codesign workshops and development sprints to test and refine the DI (Sentinel). In phase 4, we will conduct a mixed methods non-randomized study to determine the feasibility, acceptability, usability and safety of the DI (Sentinel). RESULTS: We expect data to be collected from up to 50 FRs for feasibility testing. We expect to conduct approximately 30 qualitative interviews with FRs who engaged in the Sentinel trial and 20 health and social care professionals who referred FRs to the study. CONCLUSIONS: This study will provide preliminary evidence on the feasibility, acceptability, safety, usability and implementation potential of the DI (Sentinel). The findings will inform the decision to proceed with a powered efficacy trial.