We propose a hybrid wavelength selective switch (WSS) using silica and silicon waveguides and describe its design. In the proposed design, silica arrayed waveguide gratings (AWGs) are connected to a silicon photonics switch array by free space optics. The wavelength multiplexed (WDM) signal input to the silica waveguide is coupled to a polarization beam splitter for polarization diversity, and one of the output signals is rotated by a half-wave plate so that both output signals had the same polarization. Each output signal is guided to an AWG and spectrally decomposed onto grating couplers in the silicon photonics chip. The two AWGs are identical and cover the whole C-band. The free space optics has cylindrical lenses to adjust the size of the light profiles to match the size of the grating couplers. The grating couplers are aligned seamlessly to realize a flexible-grid WSS. The optical signals from the grating couplers are guided to a 1 × N Mach-Zehnder interferometer (MZI) switch, and to N output grating couplers. In this design, N was set to 2, limited by the size of the silicon photonics chip. The optical signals emitted from the output grating couplers are again coupled to the corresponding AWG. Finally, the two polarized light signals are combined and output from the silica waveguide.