The development of a cost-effective and flexible sensor for hydrogen peroxide (H2O2) detection has a fundamental importance in food and medical applications. Here, flexible paper- and hydrogel-based sensors are developed using conductive polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) combined with horseradish peroxidase (HRP). These flexible sensors demonstrated a micromolar sensitivity range towards H2O2 in both liquid and vapor phases. Continuous detection in liquid is also demonstrated between 613 nM and 6.13 mM H2O2.