Internet-delivered cognitive-behavioral therapy (iCBT) is a promising treatment for chronic pain among youth, but effect sizes are small, and strategies aimed at enhancing treatment effects are needed. Participants’ engagement with the program may be an important factor in determining treatment outcomes. The primary aim of the current study was to examine the relationship between treatment engagement and treatment outcomes. Secondarily, we sought to characterize participant engagement in an iCBT program for adolescents with chronic pain and their parents. Participants included 134 adolescents randomized to the intervention arm of a controlled trial examining iCBT for chronic pain. Overall engagement with the intervention by adolescents and parents was high. Parental engagement (number of modules completed by parents and number of parent logins) predicted adolescent activity limitations change scores at post-treatment. Contrary to our expectation, adolescent treatment engagement was not predictive of treatment outcomes. Results indicate that parental engagement with the program may be an important predictor of treatment outcomes. Further research is needed to better understand influences of treatment engagement on outcomes in iCBT for youth.