Purpose Endocrine therapies using tamoxifen and/or aromatase inhibitors are important therapeutic options for the targeted treatment of hormone-responsive breast cancer. In addition to nuclear estrogen receptors ERα and β, G-protein-coupled estrogen receptor GPER-1 is a third receptor-mediating estrogen effects in breast cancer cells. The aim of this study was to examine to what extent GPER-1 expression might affect the efficacy of primary endocrine treatment of breast cancer. Methods GPER-1 expression was determined in tissue samples from patients with early breast cancer by means of immunohistochemistry and a GPER-1 score of ≥ 3 was considered to be positive. In a total of 165 patients, the response to a primary therapy with tamoxifen (TAM) or aromatase inhibitors (AI) was assessed by ultrasound imaging for up to 6 months. The primary endpoint of this study was the response to treatment evaluated by RECIST 1.1 criteria. Results GPER-1 expression was observed in 127 (77.0%) out of 165 cases. Based on GPER-1 expression and the type of endocrine treatment, the patients were divided into 4 groups: GPER-1 negative/TAM (12.1%), GPER-1 negative/AI (10.9%), GPER-1 positive/TAM (44.8%), and GPER-1 positive/AI (32.1%). The groups were well balanced regarding different clinical and pathological factors. After 4 and 6 months of treatment, a high level of stable disease or progressive disease was observed in the GPER-1 positive/TAM group only ( p  < 0.0001), whereas in the other three groups of patients, the most common objective response was classified as partial response. We observed a continuous reduction of mean tumor size in patients treated with aromatase inhibitors irrespective of the GPER-1 status and in GPER-1 negative patients treated with TAM. In contrast, in GPER-1 positive patients treated with TAM, a reduction of mean tumor size was observed only in the first 2 months after beginning of treatment. Four and six months after start of treatment, no reduction, but even a slight increase of tumor size was observed in this patients group. Conclusions GPER-1 expression is significantly associated with a reduced effect of primary treatment with tamoxifen in breast cancer patients.