Objective: Internet-based cognitive–behavioral treatments (ICBT) have shown promise for various mental disorders, including social anxiety disorder (SAD). Most of these treatments have been delivered on desktop computers. However, the use of smartphones is becoming ubiquitous and could extend the reach of ICBT into users’ everyday life. Only a few studies have empirically examined the efficacy of ICBT delivered through a smartphone app and there is no published study on mobile app delivered ICBT for SAD. This three-arm randomized-controlled trial (RCT) is the first to compare the efficacy of guided ICBT for smartphones (app) and conventional computers (PC) with a wait list control group (WL). Method: A total of 150 individuals meeting the diagnostic criteria for SAD were randomly assigned to one of the three conditions. Primary endpoints were self-report measures and diagnostic status of SAD. Results: After 12 weeks of treatment, both active conditions showed superior outcome on the composite of all SAD measures (PC vs. WL: d = 0.74; App vs. WL: d = 0.89) and promising diagnostic response rates (NNTPC = 3.33; NNTApp = 6.00) compared to the WL. No significant between-groups effects were found between the two active conditions on the composite score (Cohen’s d = 0.07). Treatment gains were maintained at 3-month follow-up. Program use was more evenly spread throughout the day in the mobile condition, indicating an integration of the program into daily routines. Conclusions: ICBT can be delivered effectively using smartphones.