Trastuzumab (Tmab), a humanized monoclonal antibody that selectively targets human epidermal growth factor receptor 2 (HER2), is currently used in the clinical setting for the treatment of both breast and gastric cancer. While Tmab has shown improvements in patient prognoses, acquired resistance to this agent remains an issue. While some novel HER2-targeted agents have been approved for clinical use in breast cancer, no such agent has shown treatment efficacy for gastric malignancies with Tmab-resistance. Nanotechnology, which has progressed rapidly, has been applied to medical fields in recent years. Gold nanopartides, which are characterized by their in vivo stability and ease of surface modification, have been reported to show efficacy as the carriers of therapeutic agents, such as drugs, antibodies, peptides, and nucleic acids. In this work, we developed Tmab-conjugated gold nanopartides and demonstrate their efficacy for the treatment of HER2-positive, Tmab-resistant gastric cancer cell lines. Our findings demonstrate that Tmab-conjugated gold nanopartides have the potential to be a novel HER2-targeted therapeutic agent.