Metabolic activation of therapeutic agents to chemically reactive intermediates is considered to be an initiation mechanism for idiosyncratic drug reactions, thus drug bioactivation process must be addressed at an early stage in drug discovery. Therefore, the development of approaches for fast screening of reactive metabolites is of great interest. In this work, we present a new technique in a “lab-on-a-plate” format that relies on ultraviolet-induced TiO2 photocatalytic oxidation (UV/TiO2-PCO) of xenobiotic solution with simultaneous glutathione (GSH) trapping of generated reactive intermediates and subsequent matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis of GSH adducts and metabolites. Mimicking of phase I and phase II reactions is performed on a TiO2-functionalized MALDI target spot. The effectiveness of the proposed approach was evaluated using amlodipine, amiodarone, diclofenac, indapamide and two substances with unknown metabolic information as model compounds. As a result, on-plate procedure revealed ten GSH adducts with diclofenac metabolites and seven GSH conjugates with indapamide metabolites. No GSH adducts with amlodipine and amiodarone oxidation products were detected, which supported the previous data. Moreover, the proposed technique provided rich information on oxidation products of drug candidates and proved their reactivity towards GSH. Compared with UV/TiO2-PCO in a stirred TiO2 suspension and subsequent incubation with GSH, the on-plate approach resulted in higher number of detected metabolites and GSH adducts as well as lower time- and labor costs. The developed algorithm represents a promising tool for rapid and high-throughput screening for reactive metabolites.