UNSTRUCTURED Background: Generalized anxiety disorder (GAD) is a highly prevalent and severely distressing condition that can lead to functional impairments and is considered one of the most difficult anxiety disorders to treat. There is now evidence for the efficacy of both guided and unguided online interventions for GAD. To establish the usefulness of such interventions, we want to evaluate the efficacy of an online self-help program (Selfapy) for GAD. Methods/design: Patients (N=156) diagnosed with GAD between 18 and 65 years with internet access and sufficient German language skills will be recruited for this study. The intervention group (IG; N=78) will receive access to the 12-week self-help online program Selfapy. The waitlist control group (CG; N=78) will receive no intervention in the context of the study. However, both groups will be allowed to access further health care services (e.g., inpatient treatment), reflecting current routine care in Germany. Outcome measures will be assessed at baseline (T1), 6 weeks (T2), and 12 weeks (T3) after the start of the intervention. The primary outcome will be generalized anxiety symptoms and quality of life at T3. Additional outcomes include depression, work capacity, therapy-related expenses and burdens, health literacy, and side effects. We expect substantial improvements in the intervention group. We aim to evaluate the therapeutic effects and the cost-efficacy of Selfapy to assess its benefits for the German healthcare system. Discussion: Online interventions may be an important addition in the German healthcare system to reduce barriers for treatment access. Further, they may prove cost-efficacy for the treatment of GAD.