The aim of this randomized controlled trial was to investigate the effects of guided internet-based cognitive behavior therapy (ICBT) for posttraumatic stress disorder (PTSD). Sixty-two participants with chronic PTSD, as assessed by the Clinician-administered PTSD Scale, were recruited via nationwide advertising and randomized to either treatment (n = 31) or delayed treatment attention control (n = 31). The ICBT treatment consisted of 8 weekly text-based modules containing psychoeducation, breathing retraining, imaginal and in vivo exposure, cognitive restructuring, and relapse prevention. Therapist support and feedback on homework assignment were given weekly via an online contact handling system. Assessments were made at baseline, post-treatment, and at 1-year follow-up. Main outcome measures were the Impact of Events Scale — Revised (IES-R) and the Posttraumatic Stress Diagnostic Scale (PDS). Results showed significant reductions of PTSD symptoms (between group effect on the IES-R Cohen's d = 1.25, and d = 1.24 for the PDS) compared to the control group. There were also effects on depression symptoms, anxiety symptoms, and quality of life. The results at one-year follow-up showed that treatment gains were maintained. In sum, these results suggest that ICBT with therapist support can reduce PTSD symptoms significantly.