Background Shared decision making is an important component of patient‐centred care and decision aids are tools designed to support patients' decision making and help patients with depression to make informed choices. Objective The study aim was to assess the effectiveness of a web‐based decision aid for patients with unipolar depression. Design Randomized controlled trial. Setting and participants Adults diagnosed with a major depressive disorder and recruited in primary care centres were included and randomized to the decision aid (n=68) or usual care (n=79). Intervention Patients in the decision aid group reviewed the decision aid accompanied by a researcher. Outcome measures Knowledge about treatment options, decisional conflict, treatment intention and preference for participation in decision making. We also developed a pilot measure of concordance between patients' goals and concerns about treatment options and their treatment intention. Results Intervention significantly improved knowledge (P<.001) and decisional conflict (P<.001), and no differences were observed in treatment intention, preferences for participation, or concordance. One of the scales developed to measure goals and concerns showed validity issues. Conclusion The decision aid “Decision making in depression” is effective improving knowledge of treatment options and reducing decisional conflict of patients with unipolar depression. More research is needed to establish a valid and reliable measure of concordance between patients' goals and concerns regarding pharmacological and psychological treatment, and the choice made.