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Cu2SnS3 (CTS) is a medium-temperature, ecofriendly, p-type thermoelectric material known for phonon-glass-electron-crystal characteristic. In the present work, ordered and disordered CTS samples were prepared from elemental powders, and their electronic and vibrational properties were systematically investigated by experimental methods and ab initio calculations. The disordered CTS polymorph presents a higher power factor, PF ∼ 1.5 μW/K2 cm, than the ordered and stable phase, PF ∼ 0.5 μW/K2 cm, above 700 K, as an effect of a smaller band gap and higher carrier concentration. Most importantly, the disordered CTS shows an ultralow thermal conductivity, k ∼ 0.4–0.2 W/m K, as compared to ordered, k ∼ 1.0–0.4W/m K, in the temperature range of 323–723 K. The combined effect of a higher PF and lower k results in a higher figure of merit, zT ∼ 0.5 at 723 K, obtained for disordered CTS without resorting to chemical alloying. It turns out that structural disorder contributes to the suppression of thermal conductivity. While group velocity of acoustic phonons, as shown both by experiments and ab initio calculations, is similar in the two polymorphs, a strong anharmonicity characterizes the disordered CTS, resulting in the presence of low-lying optical modes acting as traps for heat transmission. Density functional theory/density functional perturbation theory simulations and nuclear inelastic scattering combined with high-resolution diffraction studies of the lattice parameters reveal details of phonon–phonon interactions in CTS with unprecedented effectiveness.