Unlike graphene flakes which are semi-metallic with zero bandgaps, graphene nanoribbons (GNRs) are semiconductors with a sizeable bandgap generated by lateral quantum confinement. In this work, semiconducting GNRs are synthesized to fabricate thermoelectric generators (TEGs), which have been envisaged theoretically but are yet to be demonstrated experimentally to date. The GNRs are produced by unzipping single-walled carbon nanotubes through a mild chemical rection and ultrasonic process in solution. The semiconducting nature of the GNRs is demonstrated by strong, sharp emission of photoluminescence at 685 nm, corresponding to a bandgap of 1.81 eV, and further confirmed by on/off ratios up to 400 in the GNR-based field-effect transistors. The GNR TEGs exhibit a high Seebeck coefficient of 68 μVK−1 and a power factor of 6.76 μWm−1K−2 at room temperature, which are amongst the best in carbon-based thermoelectric devices. Finally, tests also demonstrate that the GNR TEGs are mechanically robust and flexible, showing promise in thermoelectric applications for flexible electronics.