BackgroundDespite the existence of evidence-based therapy options for the treatment of chronic pain and comorbid depressive symptoms (e.g., CBT), many individuals remain untreated. Treatment gaps result from a lack of specialists, patient fear of stigmatization, or patient immobility. Internet-based self-help interventions could serve as an anonymous and flexible alternative treatment option. In a pilot study, chronic pain patients with comorbid depressive symptoms who used a generic Internet-based depression program showed a significant reduction in depressive symptoms (but not pain symptoms) compared to a waitlist control group. Based on these findings, we developed the low-threshold, anonymous, and cost-free Internet-based self-help intervention Lenio that is tailored to the specific needs of chronic pain patients with comorbid depressive symptoms. Lenio is accompanied by the smartphone application (app) COGITO designed to increase therapeutic success. With Lenio and COGITO addressing both chronic pain and depressive symptoms, the trial aims to increase treatment effects of online interventions for chronic pain patients by reducing both depressive symptoms and pain.MethodsThe effectiveness of the Internet-based self-help intervention and accompanying smartphone app will be evaluated in a randomized controlled trial (RCT). A total of 300 participants will be randomized into an intervention group (Lenio/COGITO), an active control group (depression-focused smartphone app), or a waitlist control group. Assessments will be done at baseline, after an 8-week intervention period (post), and after 16 weeks (follow-up). The primary outcome is the post-assessment reduction in “pain impairment” (mean value of impairment in daily life, free time, and work) as assessed by the DSF (German pain questionnaire).Secondary outcomes will include the reduction in depressive symptoms as well as in the severity of pain.DiscussionLenio is one of the first Internet-based interventions to reduce chronic pain and depression that will be empirically evaluated. Internet-based interventions could offer a promising alternative to conventional face-to-face psychotherapy in the treatment of chronic pain patients. The primary objective of the current study is to add essential insight into the feasibility, effectiveness, and acceptance of Internet-based interventions for people with chronic pain and depressive symptoms.Trial registrationDRKS-ID DRKS00026722, Registered on October 6th, 2021.