We demonstrate dual-junction (Al)GaInP/GaAs solar cells designed for operation at 400 °C and 1000x concentration. For the top junction, we compare (Al)GaInP solar cells with room-temperature bandgaps ranging from 1.9 to 2.0 eV. At 400 °C, we find that ∼1.9 eV GaInP solar cells have a higher open-circuit voltage and a lower sheet resistance than higher bandgap (Al)GaInP solar cells, giving them a clear advantage in a tandem configuration. Dual-junction GaInP/GaAs solar cells are fabricated, and we show temperature-dependent external quantum efficiency, illuminated current–voltage, and concentrator measurements from 25 °C to 400 °C. We measure a power conversion efficiency of 16.4% ± 1% at 400 °C and 345 suns for the best dual-junction cell, and discuss multiple pathways to improve the performance further. After undergoing a 200 h soak at 400 °C, the dual-junction device shows a relative loss in efficiency of only ∼1%.