AbstractMultidrug resistance (MDR) is a primary limitation of breast cancer chemotherapy. The common mechanism of MDR is various anticancer drugs can be effluxed by the cell membrane protein P‐glycoprotein (P‐gp). Here, we found that ectopic overexpression of Shc3 was detected specifically in drug‐resistant breast cancer cells, consequently reducing sensitivity to chemotherapy and promoting cell migration by mediating P‐gp expression. However, the molecular mechanism underlying the interplay between P‐gp and Shc3 in breast cancer is unknown. We reported an additional resistance mechanism involving an increase in the active form of P‐gp after Shc3 upregulation. MCF‐7/ADR and SK‐BR‐3 cells can be sensitive to doxorubicin after knockdown of Shc3. Our results indicated that the interaction between ErbB2 and EphA2 is indirect and regulated by Shc3, and also, this complex is essential for activation of the MAPK and AKT pathways. Meanwhile, Shc3 promotes ErbB2 nuclear translocation, followed by a subsequent increase of the COX2 expression through ErbB2 binding to the COX2 promoter. We further demonstrated that COX2 expression was positively correlated with P‐gp expression and the Shc3/ErbB2/COX2 axis upregulates P‐gp activity in vivo. Our results show the crucial roles of Shc3 and ErbB2 in modulating P‐gp efficacy in breast cancer cells and suggest that Shc3 inhibition may enhance the sensitivity to chemotherapeutic drugs that target oncogene addiction pathways.