Purpose: To confirm the efficacy and safety of a novel ophthalmic cyclosporine A gel (CyclAGel, 0.05% CsA) in treating patients with moderate-to-severe dry eye disease (DED). Patients and Methods: The COSMO trial was a randomized, multicenter, double-masked, vehicle-controlled, phase III trial. Patients with moderate-to-severe DED were enrolled in 37 hospitals in China between November 2020 and April 2021. Eligible patients were randomized 1:1 to receive CyclAGel 0.05% or vehicle eye drops once nightly (QD). The primary endpoint was the proportion of subjects with at least a 1-point improvement in ICSS at day 84. Treatment-emergent adverse events (TEAEs) were recorded. Results: The full analysis set (FAS) included 315 and 312 participants in the CyclAGel and vehicle groups, respectively. The primary efficacy endpoint was achieved. The proportion of subjects with at least a 1-point improvement in ICSS from baseline to day 84 was significantly higher in the CyclAGel group than in the vehicle group (73.7% [232/315] vs 53.2% [166/312], P<0.0001). Significant improvements relative to the vehicle were also observed in the ICSS and Oxford CyclAGel group than in the vehicle group on days 14 and 84 (all P<0.05). The CyclAGel 0.05% was well tolerated, and the TEAEs were mostly mild. The most frequent treatment-related TEAE was eye pain (6.9% vs 1.6% in the CyclAGel and vehicle groups, respectively). No serious treatment-related TEAEs were reported. Conclusion: Clinically and statistically significant improvements in ICSS, tear production, and symptoms were observed in participants administered CyclAGel 0.05% QD for moderate-to-severe DED. CyclAGel 0.05% QD is a new effective, safe, and well-tolerated therapeutic option that might bring additional benefits of convenience and compliance as a once-A-day treatment for DED.