Exosomes are small extracellular vesicles that can be utilized as noninvasive biomarkers for diagnosis and treatment of cancer and other diseases. This study reports on a strategy involving a hybridized chain reaction-amplified chain coupled with an alkaline phosphatase-induced Ag-shell nanostructure for the ultrasensitive and rapid surface-enhanced Raman scattering immunoassay of exosomes. Exosomes from prostate cancer were captured using prostate-specific membrane antigen aptamer-modified magnetic beads; then, the hybridized chain reaction-amplified chain was released, incorporating a large number of functional moieties with signal amplification effects. Moreover, the steps of traditional immunoassay were simplified using magnetic materials, and the rapid, sensitive, and accurate detection of exosomes was achieved. Results could be obtained within 40 min with a detection limit of 19 particles/μL. Furthermore, the sera of human prostate cancer patients could be easily distinguished from those of healthy controls, highlighting the potential use of exosome analysis in clinical diagnostics.