The simple, rapid, sensitive, and specific detection of cancer cells plays a pivotal role in the diagnosis and prognosis of cancer. We developed a novel DNA-labeled sandwich electrochemical biosensor based on a glassy carbon electrode modified with 3D graphene and a hybrid of Au nanocages (Au NCs)/amino-functionalized multiwalled carbon nanotubes (MWCNT-NH₂) for label-free and selective detection of MCF-7 breast cancer cells via differential pulse voltammetry. The layer-by-layer assembly and cell-detection performance of the Au NCs/MWCNTs-NH₂-based biosensor were investigated using scanning electron microscopy and electrochemical methods including cyclic voltammetry and electrochemical impedance spectroscopy. Owing to the advantages of DNA-labeled antibodies and a nanomaterial-based signal amplification strategy, the fabricated cytosensor exhibited high specificity and sensitivity when detecting MCF-7 cells in the range of 1.0 × 10² to 1.0 × 10⁶ cells mL^-1 with a low detection limit of 80 cells mL^-1 (3σ/slope). Furthermore, the biosensor exhibited high selectivity when detecting MCF-7 cells and showed considerable potential for practical applications. The proposed DNA-labeled sandwich electrochemical biosensor provides a stable, sensitive approach to detecting cancer cells and is promising in terms of potential applications to cancer diagnosis.