A novel surface-enhanced Raman spectroscopy (SERS) sensor was proposed for an ultrasensitive detection immunoassay based on tyramine signal amplification (TSA) strategy. In this study, an immune sandwich was prepared with a capture antibody and a horseradish peroxidase (HRP)-conjugated antibody upon the addition of a target antigen. In the presence of HO, HRP can convert tyramine to a short-lived radical intermediate that forms covalent compounds with aromatic amino acids on the surfaces of proteins. By labeling the tyramine with SERS tags in the form of gold nanoparticles (AuNPs) functionalized with a Raman-active probe (4-mercaptobenzoic acid, 4-MBA), AuNPs@4-MBA was deposited and aggregated near the proteins, so the SERS signal of 4-MBA could be detected and amplified. On the basis of the TSA strategy, the developed SERS-based immunoassay can discriminate concentrations as low as 0.01 ng/mL of the target antigen and exhibited approximately 10 times stronger SERS signal intensity than traditional SERS-based immunoassays. These results demonstrated the application potential of this TSA-based SERS biosensor for the detection of important proteins in biomedical research.