Objective: Internet-based cognitive-behavioral therapy (ICBT) has received increased attention as an innovative approach to improve access to evidence-based psychological treatments. Although the efficacy of ICBT for social anxiety disorder has been established in several studies, there is limited knowledge of its effectiveness and application in clinical psychiatric care. The purpose of this study was to evaluate the effectiveness of ICBT in the treatment of social anxiety disorder and to determine the significance of patient adherence and the clinic's years of experience in delivering ICBT. Method: A longitudinal cohort study was conducted using latent growth curve modeling of patients (N = 654) treated with ICBT at an outpatient psychiatric clinic between 2009 and 2013. The primary outcome measure was the Liebowitz Social Anxiety Scale-Self-Rated. Results: Significant reductions in symptoms of social anxiety were observed after treatment (effect size d = 0.86, 99% CI [0.74, 0.98]). Improvements were sustained at 6-month follow-up (d = 1.15, 99% CI [0.99, 1.32]). Patient adherence had a positive effect on the rate of improvement. A positive association between the clinic's years of experience with ICBT and treatment outcome was also observed. Conclusions: This study suggests that ICBT for social anxiety disorder is effective when delivered within the context of a unit specialized in Internet-based psychiatric care and may be considered as a treatment alternative for implementation within the mental health care system.