A polarization-switchable single photon source is demonstrated by embedding a self-assembled quantum dot in a high-quality, electrically gated, oxide-apertured micropillar cavity. Due to the noncircular aperture, the polarization degeneracy of the fundamental cavity mode is lifted, leaving two linearly polarized Q≈20 000 modes separated by 194 μeV. An intracavity electric field generated by an applied bias enables Stark shift tuning of the quantum dot emission over a frequency range containing both polarization modes, switching the dominant single photon polarization through the Purcell effect. We measure polarization switching up to 300 kHz, limited by the RC time constant of the device.