Background Appropriate treatment for youth depression is an important public health priority. Shared decision making has been recommended, yet no decision aids exist to facilitate this. Objectives The main objective of this study was to evaluate an online decision aid for youth depression. Design An uncontrolled cohort study with pre‐decision, immediately post‐decision and follow‐up measurements. Setting and Participants Young people (n=66) aged 12‐25 years with mild, mild‐moderate or moderate‐severe depression were recruited from two enhanced primary care services. Intervention Online decision aid with evidence communication, preference elicitation and decision support components. Main outcome measures The main outcome measures were ability to make a decision; whether the decision was in line with clinical practice guidelines, personal preferences and values; decisional conflict; perceived involvement; satisfaction with decision; adherence; and depression scores at follow‐up. Results After using the decision aid, clients were more likely to make a decision in line with guideline recommendations (93% vs 70%; P=.004), were more able to make a decision (97% vs 79%; P=.022), had significantly reduced decisional conflict (17.8 points lower (95% CI: 13.3‐22.9 points lower) on the Decisional Conflict Scale (range 0‐100)) and felt involved and satisfied with their decision. At follow‐up, clients had significantly reduced depression symptoms (2.7 points lower (95% CI: 1.3‐4.0 points lower) on the Patient Health Questionnaire nine‐item scale (range 0‐27)) and were adherent to 88% (95% CI: 82%‐94%) of treatment courses. Discussion and Conclusions A decision aid for youth depression can help ensure evidence‐based, client‐centred care, promoting collaboration in this often difficult to engage population.