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Background and objectives: Clients' knowledge about their condition and treatment is considered crucial for general health improvement, and knowledge acquisition is an essential part in internet-based cognitive behavioural therapy (ICBT). Yet, little is known about the role of knowledge and how it influences treatment outcome. This study aimed to examine if explicit knowledge increased following ICBT for adolescent depression, if knowledge gain would be associated with symptom reduction, and if pre-existing knowledge predicted changes in depressive symptoms. Methods: Seventy-one adolescents were randomised to a therapist-supported ICBT or a attention control condition. A measure of depression (BDI-II) and a knowledge test dealing with depression, comorbid anxiety, and its CBT-treatment were administered before and after treatment. Results: Significant improvements in knowledge were observed following ICBT compared to the attention control (between-group Cohen's d = 1.25, 95% CI [0.67–1.79]). On average, participants in the treatment group answered 1.4 more questions correctly at post treatment compared to the control group. No relation between change in knowledge and change in depressive symptoms could be observed. Knowledge scores at baseline were high for both groups, with participants answering approximately 75% of the questions correct. A higher level of initial knowledge level predicted poorer treatment response (Parson's r = −0.38, p =.048). Conclusions: The findings indicate that knowledge about basic concepts and principles about depression, anxiety, and CBT increases following ICBT. This increase in knowledge was not related to change in depressive symptoms, indicating that knowledge is a different construct. The results also suggest that clients who are more knowledgeable prior to treatment might benefit less from ICBT. In sum, the results highlight the need to further examine the role of knowledge in ICBT. © 2018 The Authors