More than 1 million women worldwide are diagnosed with breast cancer (BC) each year. This study aims to explore the molecular mechanisms of β-catenin affecting the trastuzumab tolerance in HER2-positive BC. β-catenin in BC and non-BC tissue samples were assessed by immunohistochemistry. β-catenin and HER2 were over-expressed and knockdown to evaluate their role in tumorigenicity and trastuzumab resistance in cell and animal models using soft-agar and xenograft assays. Confocal laser immunofluorescence assay and co-immunoprecipitation were used to assess protein-protein binding. Expression of genes was detected using Western blot analysis. β-catenin was highly expressed in primary and metastatic BC, overexpression of β-catenin increased the colony formation of MCF7 cells when it was co-expressed with HER2 and synergically increased the tumor size in immunodeficient mice. Overexpression of β-catenin also increased the phosphorylation of HER2 and HER3 and increased the size of tumor derived from HER2-elevated cells. Confocal laser immunofluorescence assay showed that β-catenin and HER2 were co-localized on the membrane of MDA-MB-231 cells, suggesting that β-catenin binds HER2 to activate the HER2 signaling pathway. Immunoprecipitation of β-catenin and HER2 also confirmed this binding. On the other hand, knockdown of β-catenin in MDA-MB-231 cell lines decreased the activity of SRC and decreased phosphorylation of HER2 at Y877 and Y1248. The interaction between HER2 and SRC was enhanced when β-catenin was overexpressed, and β-catenin increased the resistance of tumor derived from HER2 elevated BT474 cells to trastuzumab. Further analysis showed that trastuzumab inhibited the activation of HER3, but SRC was still highly expressed in cells overexpressing β-catenin. Our work demonstrates that β-catenin is highly expressed in BC and it synergically promotes formation and progress of BC with HER2. β-catenin binds with HER2 leading to enhanced interaction with SRC and resistance to trastuzumab.