Evidence-based psychological interventions for adults with attention deficit hyperactivity disorder (ADHD) are seldom available in clinical settings. Medication is often offered as the sole treatment, with non-optimal effects for a majority of patients. The objective was to compare internet-based cognitive behavioral therapy (iCBT) to an active control treatment of internet-based applied relaxation training (iART), and to treatment as usual only (TAU) in adult outpatients with ADHD. One hundred and four patients, of which 67 % used ADHD medication, were randomized to 12 weeks of iCBT (n = 36), iART (n = 37), or TAU (n = 31). Primary outcome was change in the Adult ADHD Self Report Scale (ASRS) up to 3 (FU3) and 12 months (FU12) after treatment. ASRS improved more for iCBT (p < .01; Cohen's d = 0.42 at post-treatment and 0.67 at FU3) and iART (p < .01; Cohen's d = 0.57 at post-treatment and 0.66 at FU3) than for TAU. The effects sustained over 12 months for iCBT (p < .001) and iART (p < .001). No significant difference was found when comparing iCBT to iART (p = .53). Treatment responders reached 25 % for both treatments, which was superior to the 3 % responders in TAU (p < .05). iCBT and iART could both be promising add-ons to medication and increase availability to psychological treatment with sustained symptom reductions after one year.