In this study, a simple and highly selective homogeneous sandwich assay was developed for fast and ultrasensitive detection of the tau protein using a combination of monoclonal anti-tau functionalized hybrid magnetic nanoparticles and polyclonal anti-tau immobilized gold nanoparticles as the recognition and surface-enhanced Raman scattering (SERS) component, respectively, with an ability to be collected using a magnetic field. The magnetic silica particles were first coated with poly(2-hydroxyethyl methacrylate) via surface-mediated RAFT polymerization and then biofunctionalized with monoclonal anti-tau, which are both specific for tau and can be collected via a simple magnet. After separating tau from the sample matrix, they were sandwiched with the SERS substrate composed of polyclonal anti-tau and 5,5-dithiobis(2-dinitrobenzoic acid) on gold nanoparticles. The correlation between the tau concentration and SERS signal was found to be linear within the range of 25 fM to 500 nM. The limit of detection for the sandwich assay is less than 25 fM. Moreover, the sandwich assay was also evaluated for investigating the tau specificity on bovine serum albumin and immunoglobulin G.