Importance Although goniotomy is known to be successful in treating congenital glaucoma, its effect in adult glaucoma patients remains unclear. Background To evaluate the efficacy and safety of goniotomy performed simultaneously with cataract surgery in treatment of open‐angle glaucoma (OAG). Design Retrospective comparative study. Participants A total of 76 patients with moderately controlled OAG (intraocular pressure [IOP] ≤ 21 mmHg using medications) undergoing cataract surgery. Methods Comparison of patients who underwent the conventional goniotomy during cataract surgery (combined goniotomy group) with those who underwent cataract surgery alone (phaco group). Main Outcome Measures Changes in IOP and medications, and complications through 12 months. Results Baseline IOP was 18.2 ± 2.4 mmHg in the combined goniotomy group and 17.4 ± 1.9 mmHg in the phaco group; number of medications was 2.6 ± 1.1 and 2.4 ± 0.9, respectively (P > 0.05). The reduction in IOP and medication use from baseline in the combined goniotomy group was significantly greater at 12 months compared to the phaco group (−3.1 ± 2.9 mmHg vs −1.3 ± 2.4 mmHg and −1.2 ± 0.9 vs −0.7 ± 0.9, respectively, both P < 0.05). The success rate was 76.7% in the combined goniotomy group and 50.0% in the phaco group at 12 months (P = 0.021). No significant complication was observed in either group. Conclusions and Relevance Combined goniotomy and cataract surgery showed a significantly greater reduction in IOP and number of medications compared to cataract surgery alone at 1 year after surgery, with similarly favourable safety profiles.