A single natural cotton fiber has been functionalized with poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) conductive polymer by a simple soaking process and used as a channel of an organic electrochemical transistor (OECT), directly interfaced with a liquid electrolyte in contact with an Ag wire gate. The device shows a stable and reproducible current modulation and has been demonstrated to be very effective for electrochemical sensing of NaCl concentration in water. The single wire cotton fiber OECT results to be a simple and low cost device, which is very attractive for wearable electronics in fitness and healthcare.