Companion diagnostics (CDx) represent a new frontier in personalized medicine that promises to improve treatment outcomes by matching therapies to patients. Currently, these tests are limited in scope and cannot report on real-time changes associated with disease progression and remediation. To address this, we have developed the first photoacoustic imaging-based CDx (PACDx) for the selective detection of elevated glutathione (GSH) in lung cancer. Since GSH is abundant in most cells, we utilized a physical organic approach to precisely tune the chemical reactivity to distinguish between normal and pathological states. In blinded studies, PACDx was applied to identify mice bearing lung tumors. Moreover, we designed a matching prodrug, PARx, that utilizes the same mechanism to release a chemotherapeutic with a PA readout. We demonstrate that PARx can inhibit tumor growth without off-target toxicity in a lung cancer xenograft model. We envision that this work will establish a new standard for personalized medicine by employing a unique imaging-based approach.