We report on organic electrochemical transistors (OECTs) with active channels made of hybrid inkjet-printed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and electropolymerized polyaniline (PANI) layers, exhibiting simultaneously improved electrical and pH sensing characteristics. The aniline electropolymerization with an optimum 6-cycles of cyclic voltammetry forms a porous PANI microstructured layer on the PEDOT:PSS film, resulting in high signal linearity and sensitivity of about 100 mV/pH and 20 μA/pH. The electrochemical impedance spectroscopy analysis demonstrates a 9X higher-change of interfacial capacitance when decreasing the pH with the hybrid PANI-PEDOT:PSS layer, in comparison to a bare PEDOT:PSS layer. The simple fabrication process and the high signal amplification pave the way for flexible and higher-performance pH-sensitive OECTs. These scalable devices, combined with ion-selective OECTs, would lead to a novel tool for multi-parametric analysis in different biofluids.