Cu-In-Te ternary chalcogenides have unique crystal and band structures; hence they have received much attention in thermoelectrics. In this work we have observed an enhancement in Hall carrier concentration (n H) and ultralow lattice thermal conductivity (�� L) when Cu was added to ternary Cu2.5+�� In4.5Te8 (�� = 0-0.15) compounds. The enhancement in n H is attributed to a degenerate impurity band at the G point in the valence band maximum (VBM), while the extremely low �� L results from the increased lattice disorder. We thus obtained the minimum �� L value of only 0.23 W K-1 m-1 in the sample at �� = 0.1 and 820 K, which is in good agreement with the calculation using the Callaway model. The highest thermoelectric figure of merit ZT is 0.84 for the material at �� = 0.1, which is about 0.38 higher than that of the pristine Cu2.5In4.5Te8.