Ge doped Cu3SbSe4 semiconductors with Cu deficiencies were synthesized by melting and spark plasma sintering for the investigation of their thermoelectric transport properties. Ge atoms can successfully substitute Sb lattice sites, and result in lattice shrinkage with increasing Ge content. Doping with Ge not only improves the electrical conductivity but also optimizes the power factor of Cu2.95(Sb1−xGex)Se4. Ge doped specimens also reveal a competition between alloying effects and increased carrier thermal conductivity. The zT enhancement of Ge doped alloys is mainly ascribed to the enhanced power factor and the reduced lattice thermal conductivity. As a result, Cu2.95(Sb0.96Ge0.04)Se4 reaches a maximum zT of 0.7 at 640 K, showing an approximately 35% enhancement over the pristine Cu2.95SbSe4.