Epithelial ovarian cancer (EOC) is one of the most common and lethal gynecological cancers. Novel therapeutic agents have been developed for EOC, but patient survival remains poor. Trastuzumab has been approved for breast and gastric cancers with high expression of human epidermal growth factor receptor 2 (HER2), but it has not achieved any clinical success in EOC. Dysregulated Wnt/beta-catenin signaling is involved in cancer development, but whether it plays a role in EOC resistance to trastuzumab remains largely unknown. Here, we observed that high expression of Wnt3a, beta-catenin and TCF7L2, which can form a signaling axis in the Wnt/beta-catenin pathway, commonly existed in HER2-positive EOC tissue samples and was correlated with a poor patient prognosis. Cell proliferation and migration assays and nude mouse xenograft model experiments demonstrated that the Wnt3a/beta-catenin/TCF7L2 signaling axis promoted tumor cell growth and metastasis and reduced tumor sensitivity to trastuzumab. Analysis of downstream Akt signaling suggested that the function of the Wnt3a/beta-catenin/TCF7L2 signaling axis was mediated, at least in part, through increasing Akt phosphorylation. Overall, this study reveals a crucial role for the Wnt3a/beta-catenin/TCF7L2 signaling axis in EOC resistance to trastuzumab and the potential application of HER2-targeted drugs combined with inhibitors of this signaling axis for EOC treatment. (C) 2020 The Author(s). Published by Elsevier Inc.