There is a growing body of research on diabetes-depression comorbidity that demonstrates how depression negatively affects quality of life, treatment adherence and diabetes outcomes. Diabetes patients, however, have limited access to mental health services. Therefore, depression often remains untreated. Web-based therapy could potentially be an effective way to improve access to psychological care for diabetes patients. This paper describes an 8-week self-help course for adults with diabetes with comorbid depression. The intervention builds on an existing course that has shown to be effective to help improve mood in the general population. Furthermore, it describes how this course will be tested in a two-arm randomised trial, using a wait-list controlled design. It is concluded that the internet intervention, when it proves to be effective in reducing depression and improving diabetes self-management, could be disseminated to reach large groups of diabetes patients with concurrent depressive symptoms. © 2009 Bohn Stafleu van Loghum.