Acceptance and Commitment Therapy (ACT) can be effective in treating anxiety disorders, yet there has been no study on Internet-delivered ACT for social anxiety disorder (SAD) and panic disorder (PD), nor any study investigating whether therapist guidance is superior to unguided self-help when supplemented with a smartphone application. In the current trial, n = 152 participants diagnosed with SAD and/or PD were randomized to therapist-guided or unguided treatment, or a waiting-list control group. Both treatment groups used an Internet-delivered ACT-based treatment program and a smartphone application. Outcome measures were self-rated general and social anxiety and panic symptoms. Treatment groups saw reduced general (d = 0.39) and social anxiety (d = 0.70), but not panic symptoms (d = 0.05) compared to the waiting-list group, yet no differences in outcomes were observed between guided and unguided interventions. We conclude that Internet-delivered ACT is appropriate for treating SAD and potentially PD. Smartphone applications may partially compensate for lack of therapist support.