This paper presents a continuous process for printing flexible thin-film thermoelectric devices via a roll-to-roll (R2R) system. Poly (3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) was used as inks for printing p-type strips and connection strips. Nitrogen-doped graphene was dispersed into solvents to serve as n-type inks for printing n-type strips. The substrate and roller surfaces were treated for desired ink-wettability. With tunable rotation pressure and speed, the inks were transferred and printed to the flexible plastic films for horizontal thermoelectric devices. At a temperature gradient of 10 °C, the thin-film thermoelectric devices could generate an electric power of ~0.24 mW/m2 and the maximum voltage reached 3 mV, indicating great potentials for practical applications.