Ionic thermoelectric materials, for example, polyelectrolytes such as polystyrene sulfonate sodium (PSSNa),constitute a new class of materials which are attracting interest because of their large Seebeck coefficientand the possibility that they could be used in ionic thermoelectric SCs (ITESCs) and field effect transistors.However, pure polyelectrolyte membranes are not robust or flexible. In this paper, the preparation of ionicthermoelectric paper using a simple, scalable and cost-effective method is described. After a compositewas fabricated with nanofibrillated cellulose (NFC), the resulting NFC–PSSNa paper is flexible andmechanically robust, which is desirable if it is to be used in roll-to-roll processes. The robust NFC–PSSNa thermoelectric paper combines high ionic conductivity (9 mS cm1), high ionic Seebeckcoefficient (8.4 mV K1) and low thermal conductivity (0.75 W m1 K1) at 100% relative humidity,resulting in overall figure-of-merit of 0.025 at room temperature which is slightly better than that for thePSSNa alone. Fabricating a composite with cellulose enables flexibility and robustness and this is anadvance which will enable future scaling up the manufacturing of ITESCs, but also enables its use fornew applications for conformable thermoelectric devices and flexible electronics.