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Purpose: This article focused on STING agonist role in breast cancer (BCa) immunotherapy.Methods: Clinical samples were collected from 37 BCa patients'. Xenograft tumor model was established by injecting 4T1 cells into mammary fat pad of mice. STING agonist and Atezolizumab were injected into mice with 2 times a week for 2 weeks. Peripheral blood, xenograft tumor, lung, liver, brain-cortex and kidney of mice were collected. Anti-IFNAR1 was used to treat 4T1 cells. Quantitative reverse transcription-polymerase chain reaction and Western blot were used for mRNA and proteins expression. Enzyme-linked immunosorbent assay, immunohistochemistry and hematoxylin-eosin staining were performed. Results: Tumor tissues of BCa patients exhibited lower STING and high PD-1 and PD-L1 protein expression. STING agonist inhibited 4T1 cells growth in mice (P < 0.001). STING agonist increased IFNβ level and the phosphorylation of STING, TBK1, IRF3 and STAT1 in xenograft tumor (P < 0.001). STING agonist synergized with Atezolizumab to inhibit 4T1 cells growth in mice, and increase TNF-α, IFN-β and IL-10 level in peripheral blood and xenograft tumor (P < 0.01). STING agonist synergized with Atezolizumab to increase CD8+ cytotoxic T cells and decrease FOXP3+ Tregs cells in xenograft tumor. STING agonist was non-toxic to lung, liver, brain-cortex and kidney. Anti-IFNAR1 reversed STING agonist promotion on TBK1, IRF3 and STAT1 phosphorylation in 4T1 cells (P < 0.01).Conclusion: STING agonist enhances the efficacy of Atezolizumab in BCa immunotherapy by activating the IFNβ signaling pathway.