Attention deficit hyperactivity disorder (ADHD) is a disorder characterized by hyperactivity, impulsivity and lack of attention. It can have a major impact on everyday life and result in negative consequences for one's personal, academic, and work situation. For individuals with symptoms of ADHD, increased levels of anxiety and depression are common, and an overall reduction of quality of life is often present. This study protocol describes a clinical trial of internet-based cognitive behavioral therapy (iCBT), using a randomized controlled study design, with the primary aim to increase quality of life, as well as to reduce symptoms of ADHD, anxiety, depression, and stress. A second aim is to investigate, by qualitative means, what aspects of treatment were perceived as helpful and hindering when it comes to completing iCBT. Two hundred participants with symptoms of ADHD will be included and randomized to two conditions (treatment and wait-list control). The treatment period is comprised of a maximum of ten weeks, with two mandatory modules and ten modules from which the participants can choose freely. Self-report measures will be completed by the participants at baseline and end of treatment, as well as at a six-month follow-up. The treatment is guided by therapists and consists of weekly correspondence with the participants. The study will utilize an intention to treat design, with ANOVAs and Reliable Change Index to evaluate treatment effects. The qualitative part of the project will be interview-based and employ thematic analysis. Lastly, a psychometric evaluation of a common instrument for determining ADHD-symptoms will also be made. The results will hopefully contribute to the evidence base for iCBT for individuals with symptoms of ADHD and help disseminate potentially effective interventions.