The efficacy of Internet-based treatments and bibliotherapy for Social Anxiety Disorder is well established. The next step is to evaluate smartphone supported interventions. ‘Challenger’ is a recently developed facilitating the implementation of exposure exercises in daily life. The present study evaluates the efficacy of the combination of Challenger and bibliotherapy for Social Anxiety Disorder. A total of 209 patients with Social Anxiety Disorder were randomly allocated to three groups. The first group received app and bibliotherapy for six weeks (parallel design) and the second group received first bibliotherapy for six weeks and then the app for another six weeks (sequential design). The third group was a wait-list control group. Participants filled in questionnaires on primary and secondary outcomes post treatment as well as at 4 and 12 months follow-up. Participants using both app and bibliotherapy showed somewhat larger reductions in social fears after six weeks of treatment compared to participants using only bibliotherapy (d=0.25). Decreases in social anxiety were large for both groups after 12 weeks of treatment (d= 1.11-1.20) and were maintained throughout the follow-up period. Results of the current study support the notion of adding a smartphone app to bibliotherapy for Social Anxiety Disorder. Future study should investigate the potential of the app in different treatment settings.