AbstractGlassy carbon electrode was modified with palladium‐decorated conducting poly(thionine) film to develop a simple, cost‐effective, and fast amperometric sensor for the detection of H2O2. Conducting PTH film served as a porous and stable support matrix for even distribution of the Pd nanomaterials. In order to get the maximum performance from the modified electrode system, experimental conditions were optimized using chronoamperometry. The Pd/PTH@GCE electrode showed maximum performance in buffer solution at pH = 7.5 meaning it is suitable for the detection of H2O2 at physiological pH (7.4). Scanning Electron Microscopy was performed for physical characterization of the modified electrodes using pencil graphite electrodes. The active surface area for Pd/PTH‐modified GCE was calculated as 1.103 cm2. The linear concentration range was determined as 0.2–7.0 mM and its sensitivity was determined as 40.0 μA mM−1. The detection limit (LOD) of the sensor was calculated as 12.3 μM with a signal/noise ratio of 3.