Purpose. To assess the impact of the COVID-19 pandemic on adherence to oral endocrine therapy in patients diagnosed with breast cancer in the public healthcare system in Catalonia (Spain). Methods. Retrospective cohort study in patients starting endocrine therapy from 2017 to 2021. Adherence was measured during the first year of treatment, and the impact of the pandemic was calculated according to the calendar year and whether the first year of treatment included the peak period of the pandemic in our setting (March–September 2020). Analyses were performed using a chi-square test and multivariable logistic regression, with results stratified by year, age group, and drug type. Results. Mean overall adherence during the first year of treatment was 89.6% from 2017 to 2021. In contrast, the patients who started treatment in 2019 and 2020 and whose treatment included the peak pandemic period presented an adherence of 87.0% and 86.5%, respectively. Young age and tamoxifen or combination therapy were predictors of low adherence. An increase in neoadjuvant therapy was also observed in 2020. Conclusions. The COVID-19 pandemic had only a modest impact on adherence to endocrine therapy (≈3%), despite the enormous disruptions for patients, the healthcare system in general, and cancer care in particular that were occurring in that period.