With the increase in demand for low-carbon technology, green raw materials, and comfortable heating, academia and industry have paid considerable attention to cellulose-based electrothermal composites. This attention owes to the fact that cellulose is a versatile, abundant, low-cost, and sustainable material with beneficial properties. Here, we develop a novel strategy for fabricating flexible, transparent electrothermal heaters that are composed of both silver nanowire (AgNW)/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrothermal composites and a regenerated cellulose (RC) matrix. The AgNWs were spin-coated onto glass substrates and easily transferred onto an RC surface through the coagulation and regeneration of the cellulose solution. PEDOT:PSS was coated onto the AgNW-coated RC matrix to improve the electrical and electrothermal properties of the film heaters. The PEDOT:PSS/AgNW/RC composite films demonstrated an excellent optical transmittance of 73.8% at 550 nm and a low sheet resistance of 11.2 Omega/sq. These composite heaters also exhibited a rapid heating response, uniform heat distribution, excellent heat generation, and robust structural stability. These electrothermal composites made from earth-abundant, low cost, and recyclable materials have great potential for green, flexible, transparent film heaters.