Background: The evidence for the benefits of online cognitive behaviour therapy (CBT)-based programmes delivered in a clinical context is clear, but this evidence does not translate to online CBT-based stress management programmes delivered within a workplace context. One of the challenges to the delivery of online interventions is programme engagement; this challenge is even more acute for interventions delivered in real-world settings such as the workplace. The purpose of this pilot study is to explore the effect of an online facilitated discussion group on engagement, and to estimate the potential effectiveness of an online CBT-based stress management programme. Methods: This study is a three-arm randomised controlled trial (RCT) comparing a minimally guided, online, CBT-based stress management intervention delivered with and without an online facilitated bulletin board, and a wait list control group. Up to 90 employees from six UK-based organisations will be recruited to the study. Inclusion criteria will include age 18 years or over, elevated levels of stress (as measured on the PSS-10 scale), access to a computer or a tablet and the Internet. The primary outcome measure will be engagement, as defined by the number of logins to the site; secondary outcome measures will include further measures of engagement (the number of pages visited, the number of modules completed and self-report engagement) and measures of effectiveness (psychological distress and subjective wellbeing). Possible moderators will include measures of intervention quality (satisfaction, acceptability, credibility, system usability), time pressure, goal conflict, levels of distress at baseline and job autonomy. Measures will be taken at baseline, 2 weeks (credibility and expectancy measures only), 8 weeks (completion of intervention) and 16 weeks (follow-up). Primary analysis will be conducted on intention-to-treat principles. Discussion: To our knowledge this is the first study to explore the effect of an online discussion group on the engagement and effectiveness of an online CBT-based stress management intervention. This study could provide a solution to the growing problem of poor employee psychological health and begin to address the challenge of increasing engagement with Internet-delivered health interventions.