Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb)₂Te₃ bulk materials by melt-spinning single elements of Bi, Sb, and Te followed by a spark plasma sintering process. The samples that were most optimized with the resulting composition (Bi_0.48Sb_1.52Te₃) and specific nanostructures showed an increase of ∼50% or more in the figure of merit, ZT, over that of the commercial bulk material between 280 and 475 K, making it suitable for commercial applications related to both power generation and refrigeration. The results of high-resolution electron microscopy and small angle and inelastic neutron scattering along with corresponding thermoelectric property measurements corroborate that the 10-20 nm nanocrystalline domains with coherent boundaries are the key constituent that accounts for the resulting exceptionally low lattice thermal conductivity and significant improvement of ZT.