GNPs can generate highly localized heating after exposure to radiation and have been used to augment the efficacy of cancer radiotherapy to eradicate cancer cells. To promote specific tumor accumulation and reduce side effects, GNPs were conjugated to an anti-HER2 antibody to target breast cancer cells overexpressing the HER2 receptor. In addition, penetratin, a cell-penetrating peptide, was fused to the C-terminus of an anti-HER antibody without compromising its receptor binding function. The recombinant antibody was able to cross the cellular membrane more efficiently and deliver more GNPs into cancer cells. With the accumulation of GNPs in cancer cells, a much lower dose of radiation could be used to generate a similar cancer cell mortality rate. Our study offers a novel approach to engineer cell-penetrating mAbs. An effective cell-penetrating antibody could promote the delivery of GNPs into target cancer cells to augment the efficacy of cancer radiotherapy and reduce side effects in patients.