Abstract A novel transducer concept based on an organic electrochemical transistor is described. Its function as an integral part of an air humidity sensor, in which the proton conductor Nafion acts as sensitivity layer has been realised. The resulting electrochemical sensor–transistor, based on the conducting polymer PEDOT:PSS, operates at low voltages, on the order of 1 V. The sensor response, measured as the drain–source current of the electrochemical transistor, versus air humidity, has a close to exponential behaviour. The sensor can be realised using exclusively printing and coating fabrication techniques. Here, we demonstrate devices realised on plastic foils and on ordinary coated fine paper substrates. This organic electrochemical transducer promise future applications such as all-integrated low-cost sensor tags for single-use chemical sensors.