Background Intrusive memories of psychologically traumatic events bring distress both sub-clinically and clinically. This parallel-group, two-arm randomised controlled trial evaluated the effect of a brief behavioural intervention on reducing intrusive memories in frontline healthcare workers exposed to traumatic events during the COVID-19 pandemic. Methods Participants with at least two intrusive memories of work-related trauma in the week before recruitment were randomised 1:1 to an imagery-competing task intervention (n = 73) or attention-based control task (n = 71). The number of intrusive memories was assessed at baseline and 5 weeks after the guided session (primary endpoint). Results The intervention significantly reduced intrusive memory frequency compared with control [intervention Mdn = 1.0 (IQR = 0–3), control Mdn = 5.0 (IQR = 1–17); p < 0.0001, IRR = 0.30; 95% CI = 0.17–0.53] and led to fewer post-traumatic stress-related symptoms at 1, 3 and 6 month follow-ups (secondary endpoints). Participants and statisticians were blinded to allocation. Adverse events data were acquired throughout the trial, demonstrating safety. There was high adherence and low attrition. Conclusions This brief, single-symptom, repeatable digital intervention for subclinical-to-clinical samples after trauma allows scalability, taking a preventing-to-treating approach after trauma. Trial registration 2020–07-06, ClinicalTrials.gov identifier: NCT04460014.