Water-stable, flexible and micro-scale organic electrochemical transistors (OECTs) based on poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) were fabricated on a plastic substrate using a new process based on a fluorinated photoresist. The PEDOT:PSS films, mixed solely with a biocompatible conductivity enhancer, show robust adhesion on plastic substrates, and exhibit unchanged electrical properties under extreme bending. This work simplifies the fabrication of high-performance OECTs and places them in a highly competitive position for flexible electronics and healthcare applications.