A highly sensitive self-focusing surface-enhanced Raman scattering (SERS) methodology has been developed using Au@SiO₂ core–shell nanorods for carcinoembryonic antigen (CEA) detection. The SERS enhancement factor was evaluated for anisotropic Au@SiO₂ nanorods with silica shells of various thicknesses, upon which Rhodamine 6G (R6G) dye was applied as a reporter molecule for the quantitative determination of CEA. The highest R6G signal was attained with a silica layer of 1–2 nm thickness. The self-focusing character originates from the antibody–antigen interaction, which facilitates the SERS probes assembly and significantly increases the detection sensitivity of the CEA. Our results show that the SERS technique is able to detect CEA within a wide concentration range. With an extremely low limit of detection (LOD) of 0.86 fg mL⁻¹, the Au@SiO₂ nanoprobes potentially enable the early diagnosis of cancer. Our work offers a low-cost route to the fabrication of sensing devices able to be used for monitoring cancer progression in natural matrices, such as blood.