Panic disorder is a common mental disorder. Guided Internet-based cognitive behavioural therapy (Guided Internet-based cognitive behaviour therapy (ICBT)) is a promising approach to reach more people in need of help. In the present effectiveness study, we investigated the outcome of guided ICBT for panic disorder after implementation in routine care. A total of 124 patients were included in the study, of which 114 started the treatment. Large within-group effect sizes were observed on the primary panic disorder symptoms (post-treatment: d = 1.24; 6-month follow-up: d = 1.39) and moderate and large effects on secondary panic disorder symptoms and depressive symptoms at post-treatment and follow-up (d = .55–1.13). More than half (56.1%) of the patients who started treatment recovered or improved at post-treatment. Among treatment takers (completed at least five of the nine modules), 69.9% recovered or improved. The effectiveness reported in the present trial is in line with previous effectiveness and efficacy trials of guided ICBT for panic disorder. This provides additional support for guided ICBT as a treatment alternative in routine care.