The use of coherent optical signal processing in long-distance optical communication systems has dramatically increased data capacity enabling encoding of multiple-bit information in the amplitude and phase of a light beam. Direct detection of phase information of a high-speed modulated light remains challenging and requires an external, local oscillator for referencing, which is expensive for short-reach optical communications, for example, in datacenters. The availability of less complex integrated photonics devices for coherent signal detection would alleviate this bottleneck. On the other hand, phase information of coherent, orthogonally polarized light beams can be extracted from their polarization states and it is, therefore, possible to achieve phase measurements via fast polarization detection. Here we demonstrate an on-chip, high-speed coherent optical signal receiver enabled by spin-orbit coupling in Si-photonics circuitry. In a coherent communication experiment with up to 16 Gbaud/s rate, the high-speed quadrature phase-shift keying signals detected by a Si nanodisk based polarisation measurements at multiple wavelength in the C-band were recovered with a bit error rate below the forward error correction threshold. The proposed on-chip nanodisk coherent receiver shows promise in high-speed coherent optical communication applications where phase detection is required at low cost and small footprint.