GeTe is a versatile base compound to produce highly efficient p-type thermoelectric materials such as the TAGS materials (AgSbTe2)1-x(GeTe)x and GeTe-PbTe nanocomposites. The pure GeTe composition shows a very high power factor, ~42 μW cm(-1) K(-2), between 673 K and 823 K, which is among the highest power factors that have ever been reported in this temperature range. However, its relatively high thermal conductivity limits the dimensionless figure of merit ZT to values of only unity. In this paper, we present an efficient approach to reduce the thermal conductivity by preparing (In2Te3)x(GeTe)(3-3x) solid solutions. In spite of a slight degradation of the electronic properties, the drastic reduction of the thermal conductivity due to a synergistic combination of reduced electronic thermal conductivity, strong alloy scattering, and vacancy phonon scattering leads to ZT values as high as 1.35 at 823 K for the x = 0.05 sample. Our results show that (In2Te3)x(GeTe)(3-3x) is a prospective substitute for TAGS as a p-leg element for high-temperature power generation.