Background: Guided Internet-based cognitive behavior therapy (ICBT) has been tested in many trials and found to be effective in the treatment of anxiety and mood disorders. Generalized anxiety disorder (GAD) has also been treated with ICBT, but there are no controlled trials on guided Internet-based psychodynamic treatment (IPDT). Since there is preliminary support for psychodynamic treatment for GAD, we decided to test if a psychodynamically informed self-help treatment could be delivered via the Internet. The aim of the study was to investigate the efficacy of IPDT for GAD and to compare against ICBT and a waiting list control group. Method: A randomized controlled superiority trial with individuals diagnosed with GAD comparing guided ICBT (n = 27) and IPDT (n = 27) against a no treatment waiting list control group (n = 27). The primary outcome measure was the Penn State Worry Questionnaire. Results: While there were no significant between-group differences immediately after treatment on the main outcome measure, both IPDT and ICBT resulted in improvements with moderate to large within-group effect sizes at 3 and 18 months follow-up on the primary measure in the completer analyses. The differences against the control group, although smaller, were still significant for both PDT and CBT when conforming to the criteria of clinically significant improvement. The active treatments did not differ significantly. There was a significant group by time interaction regarding GAD symptoms, but not immediately after treatment. Conclusions: IPDT and ICBT both led to modest symptom reduction in GAD, and more research is needed.