Introduction Men with erectile dysfunction are often worried about their condition, have interpersonal difficulties, and have a reduced quality of life. Internet-delivered cognitive behavior therapy (ICBT) has been shown effective for a number of health problems but evidence is limited concerning the treatment of erectile dysfunction. Aim The study investigated the effects of ICBT for erectile dysfunction. Methods Seventy-eight men were included in the study and randomized to either ICBT or to a control group, which was an online discussion group. Treatment consisted of a 7-week Web-based program with e-mail-based therapist support. Each therapist spent an average of 55 minutes per participant. Main Outcome Measure The International Index of Erectile Functioning five-item version was administered via the telephone at pretreatment, post-treatment, and 6 months after receiving ICBT. Results At post-treatment, the treatment group had significantly greater improvements with regard to erectile performance compared with the control group. Between-group differences at post-treatment were small (d = 0.1), but increased at the 6-month follow-up (d = 0.88). Conclusions This study provides support for the use of ICBT as a possible treatment format for erectile dysfunction.