Although pain and function improve at immediate posttreatment for youth receiving cognitive behavioral therapy for chronic pain, limited data are available to understand changes that youth make during psychological treatment. We sought to characterize distinct trajectory patterns of change in pain and function to understand the temporal association of these changes during internet-delivered cognitive behavioral therapy (CBT). Weekly repeated assessments of pain and function were conducted during 8 weeks of treatment among 135 adolescents, aged 11 to 17 years, with chronic pain who were randomized to the cognitive behavioral intervention arm of an ongoing trial of internet-delivered CBT (Web-based management of adolescent pain; Web-MAP2). Using random-effects growth mixture models, we characterized pain and functional disability trajectories finding distinct trajectory groups indicating patterns of both linear and quadratic effects. Trajectories of change showed that some patients' pain and functional disability were improving, others worsened or changed minimally. Paired t tests compared the within-subject relative change rate in pain and function demonstrating similar change range for pain and function during the treatment period. There was no support for improvements in either pain or function to precede changes in the other domain. Findings may be useful in informing future studies of psychosocial treatments for pediatric chronic pain to consider how to target treatment strategies to distinct patient response profiles. This may lead to the development of intervention strategies that can both more effectively target children's pain and function during treatment and lead to sustained changes after treatment.