Earth-abundant quaternary chalcogenide semiconductors with complex structures, such as copper zinc tin sulphide (CuZnSnS; CZTS), have the potential to become economic and non-toxic thermoelectric materials. However, the inferior power factor of CZTS, due to its insignificant electrical conductivity, negates the advantage of inherent small thermal conductivity. In the present report, the thermoelectric properties of CZTS composites integrated with graphene nanosheets (GNs) CZTS/x (x = 0.25, 0.5, 0.75 or 1 wt% GNs) were synergistically optimized. The inclusion of GNs (⩽0.75 wt%) simultaneously enhanced the carrier transport (electrical conductivity σ) by providing conductive pathways as well as suppressed lattice thermal conductivity (κ ) due to the enhanced grain barriers in addition to interface scattering. This synergistic optimization enhanced the figure of merit, ZT, of CZTS/GN nanocomposites to its highest value (∼0.5) at 623 K for the addition of 0.75 wt% GNs, which is nearly a seven-fold enhancement over the pristine sample. The novel strategy of fabricating CZTS/GN nanocomposites by utilizing GNs is an alternative way to obtain the highest thermoelectric performance (ZT ∼ 0.5) in CZTS, and can be extended to other environmentally friendly quaternary chalcogenides.