A novel, flexible and ductile organic field-effect transistor (OFET) able to detect pH changes in chemical solutions has been realized and successfully tested. With respect to other organic pH sensors, based on an ISFET-like structure, in our approach the organic transistor is completely separated from the sensing active area and its gate is left floating. The device is biased with a fourth electrode (control-gate) capacitively coupled to the floating-gate. The floating-gate is functionalized by deposition of a layer of thio-amines able to protonize proportionally to the pH value of the solution thus modulating the drain current. The structure does not need an Ag/AgCl counter-electrode since the control-gate is not in contact with the solution. Moreover, the sensing mechanism does not depend on the choice of the dielectric and semiconductor material since the working principle is based on charge separation in the metal induced by the electric field. This structure also simplifies the realization of the fluidics since all the contactable electrodes (drain, source and control-gate) are on the same side of the substrate. A differential measurement approach was adopted in order to get rid of device aging and process-related fluctuations. With the same structure, other chemical species may be detected provided that a proper functionalization procedure is adopted.