Purpose This study aimed to evaluate the level of treatment adherence and identify predictors of adherence in glaucoma patients during the coronavirus disease 2019 (COVID-19) pandemic in Zagreb, Croatia. Methods The cross-sectional study was conducted among primary open-angle glaucoma (POAG) patients at the Department of Ophthalmology, University Hospital Centre Zagreb, between 1st April 2020 and 1st April 2021. The sociodemographic data, clinical, and drug information were recorded. The treatment adherence was assessed through a self-administered questionnaire, the Culig Adherence Scale (CAS). Statistical analyses were performed using TIBCO Statistica™ 14.0.0. Results 113 POAG patients (48M/65F) with a mean age of 65.89 ± 8.53 years were included in this study. According to the CAS, only 39.8% of patients adhered to topical glaucoma treatment. Educational level was positively related to adherence to treatment (p = 0.022). The main predictors for adherence were the patients' positive attitude towards the drug's effect, family support, and good relationship with the healthcare providers (p < 0.05, respectively). The majority of patients stated forgetfulness as the main reason for skipping drug dosing. Conclusions Non-adherence to treatment was prevalent among patients with POAG during the COVID-19 pandemic in Zagreb, Croatia. Personal beliefs and attitudes towards treatment, family support, and patient-healthcare provider relationship were significant predictors of adherence. Special consideration should be given to the whole health system-level strategies targeting the adherence to treatment, especially in crises, in order to achieve positive therapeutic outcomes. The study protocol was registered in the DRKS - German Clinical Trials Register, DRKS-ID: DRKS00022081.