Glutathione (GSH) and oxidized glutathione (GSSG) control cellular function and efficiency of anticancer therapy. Reliable detection of cellular GSH/GSSG is challenging due to their ultralow concentration (typically femtomolar concentrations) and interference by other thiol-based molecules. Here, we report a "turn-off" surface enhanced Raman scattering (SERS)-based approach for reliable detection of cellular GSH and GSSG. This approach exploits GSH-induced replacement of a Raman probe from the surface of γ-Fe2O3-Au followed by Ag growth around γ-Fe2O3-Au that generates electromagnetic hot spots at the junction between Au and Ag where the Raman probe is localized. The magnetic component of the hybrid nanoparticle concentrates the cellular GSH, and the Au/Ag-based plasmonic component provides electromagnetic hot spots for sensitive SERS. This approach is able to monitor GSH level during photothermal cancer therapy.