Liquid-like ionic conductors in the copper selenide family represent a promising class of thermoelectric materials capable of recycling waste heat into electrical energy with an exemplary figure-of-merit (zT > 1.4) above 800 K. Ion diffusion, however, is enhanced at such high temperatures and drives a non-reversible phase segregation that inhibits practical applications. In tandem, the thermoelectric performance at moderate temperatures (500–750 K) where ion diffusion is less problematic, is not optimal for real-world applications (zT < 1). In this work, we demonstrate that incorporating a small weight fraction of carbon using various carbon sources can significantly enhance the zT of Cu 2 Se at both middle and high temperatures. All the carbon-doped Cu 2 Se samples exhibit weak temperature dependent zT higher than 1.0 over a broad temperature range from 600 to 900 K, with the 0.6 wt% Super P doped Cu 2 Se sample achieving a zT of 1.85 at 900 K. Furthermore, the 0.3 wt% carbon fiber doped Cu 2 Se shows zT > 1 for T > 520 K and reaches a record level of zT of ~ 2.4 at 850 K. These values for the carbon doped Cu 2 Se are comparable or superior to those for the current state-of-the-art thermoelectric materials. Microstructure studies on graphite incorporated Cu 2 Se revealed that graphite nanostructures interspace between Cu 2 Se nanoscale grains being responsible for the significantly enhanced zT. The low thermal conductivity in the nanostructured composite is attributed to the high density of interfaces caused by the small grain diameters (30–60 nm), along with the strong acoustic mismatch between Cu 2 Se and carbon phonon states which enhances the thermal boundary resistance. This discovery indicates strong prospects for engineering carbon thermoelectric nanocomposites for a range of energy applications.