This work presents a simple, low-cost and practical inkjet-printing technique for fabricating an innovative flexible gas sensor made of graphene-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composite film with high uniformity over a large area. An electronic ink prepared by graphene dispersion in PEDOT:PSS conducting polymer solution is inkjet-printed on a transparency substrate with prefabricated electrodes and investigated for ammonia (NH3) detection at room temperature. Transmission electron microscopy, Fourier transform infrared spectroscopy, UV-visible spectrometer and Raman characterizations confirm the presence of few-layer graphene in PEDOT:PSS polymer matrix and the present of pi-pi interactions between graphene and PEDOT:PSS. The ink-jet printed graphene-PEDOT:PSS gas sensor exhibits high response and high selectivity to NH3 in a low concentration range of 25-1000 ppm at room temperature. The attained gas-sensing performance may be attributed to the increased specific surface area by graphene and enhanced interactions between the sensing film and NH3 molecules via pi electrons network. The NH3-sensing mechanisms of the flexible printed gas sensor based on chemisorbed oxygen interactions, direct charge transfers and swelling process are highlighted.