In this study, the temperature-dependent thermoelectric properties of p-type single-crystal Ge, which is a useful material for thermoelectric applications owing to its significantly high carrier mobility, were investigated. The thermoelectric properties of Ga-doped (5.7 × 10¹⁶, 3.4 × 10¹⁸, and 1.0 × 10¹⁹cm⁻³) p-type single-crystal Ge were measured from room temperature to 770 K. The sample with a carrier concentration of 1.0 × 10¹⁹cm⁻³showed the highest thermoelectric figure of merit, ZT, over the entire measured temperature range. The maximum ZT value was 0.06 at 650 K. A theoretical model based on the Boltzmann transport equation with relaxation-time approximation was developed and quantitatively reproduced the experimentally observed data. The optimal impurity concentration predicted by this model was 3 × 10¹⁹cm⁻³at 300 K and increased with temperature.