We demonstrate monolithically integrated n-GaAs/p-Si depletion-type optical phase shifters fabricated on a 300 mm wafer-scale Si photonics platform. We measured the phase shifter performance using Mach–Zehnder modulators with the GaAs/Si optical phase shifters in both arms. A modulation efficiency of VπL as low as 0.3 V·cm has been achieved, which is much lower compared to a carrier-depletion type Si optical phase shifter with pn junction. While propagation loss is relatively high at ∼6.5dB/mm, the modulator length can be reduced by the factor of ∼4.2 for the same optical modulation amplitude of a Si reference Mach–Zehnder modulator, owing to the high modulation efficiency of the shifters.