Modified forms of Bi₂Te₃ system attracts immense importance in thermoelectric generation by providing a green and clean technology for converting low-grade waste heat into electrical energy. Herein, we have explored the potential of a new material graphitic carbon nitride (g-C₃N₄) for tuning the thermoelectric properties of Bi_0·4Sb_1·6Te₃. The electronic and lattice transport properties of Bi_0·4Sb_1·6Te₃ are effectively decoupled through nanostructuring, modification of carrier concentration through defect engineering and embedding g-C₃N₄ nanostructures in the bulk material which leads to the synergetic optimization of power factor and suppressed thermal conductivity due to all-scale hierarchical architecture. All the Bi_0·4Sb_1·6Te₃/g-C₃N₄ nanocomposites exhibit reduction in thermal conductivity and we obtained an ultra-low thermal conductivity of 0.33 W/mK for Bi_0·4Sb_1·6Te₃ nanocomposite with 1% g-C₃N₄. The simultaneous optimization of electrical and lattice transport properties leads to a marked improvement in figure of merit from 0.295 for pristine Bi_0·4Sb_1·6Te₃ to a peak value of 1.09 for Bi_0·4Sb_1·6Te₃ with 0.50% g-C₃N₄.