Making decisions about disclosing a mental illness in the workplace is complicated. Decision aid tools are designed to help an individual make a specific choice. We developed a web-based decision aid to help inform decisions about disclosure for employees. This study aimed to examine the efficacy of this tool.We conducted a randomised controlled trial with recruitment, randomisation and data collection all online. Participants had access to the intervention for 2 weeks. Assessments occurred at baseline, postintervention and 6 weeks' follow-up. The primary outcome was decisional conflict. Secondary outcomes were stage and satisfaction of decision-making and mental health symptoms.107 adult employees were randomised to READY (n=53) or the control (n=54). The sample was predominantly female (83.2%). Participants using READY showed greater reduction in decisional conflict at postintervention ((1,104)=16.8, p<0.001) (=0.49, 95% CI 0.1 to 0.9) and follow-up ((1,104)=23.6, p<0.001) (=0.61, 95% CI 0.1 to 0.9). At postintervention the READY group were at a later stage of decision-making ((1,104)=6.9, p=0.010) which was sustained, and showed a greater reduction in depressive symptoms ((1,104)=6.5, p=0.013). Twenty-eight per cent of READY users disclosed, and reported a greater improvement in mental health than those who did not disclose.READY provides a confidential, flexible and effective tool to enhance employee's decision-making about disclosure. Its use led to a comparative improvement in depressive symptoms compared with the current information provided by a leading mental health non-governmental organisation, without apparent harm. READY seems worth evaluating in other settings and, if these results are replicated, scaling for wider use.ACTRN12618000229279.