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A continuously tunable photonic radio-frequency (RF) phase shifter based on polarization modulation and all-optical differentiation using a polarization modulator (PolM) and an optical frequency discriminator is proposed, theoretically analyzed and experimentally demonstrated. In the proposed phase shifter, two light waves at different wavelengths are injected into a PolM which is driven by an RF signal to be phase shifted. For one wavelength, the state of polarization (SOP) of the light wave is aligned with one principal axis of the PolM, thus the PolM operates jointly with an optical frequency discriminator as an optical differentiator to produce a quadrature (Q) signal. For the other wavelength, the SOP of the light wave is oriented with 45° or 135° with the principal axis of the PolM, thus the PolM operates jointly with a polarizer as an intensity modulator to produce an in-phase (I) signal. By detecting the I-Q optical signals at a photodetector (PD), a phase-shifted RF signal is generated. The value of the phase shift is determined by the amplitudes of the I-Q signals, which can be continuously tuned by controlling the powers of the light waves. An experiment is performed. A phase shifter with a full tunable range of 360° at an RF frequency tunable from 3 to 10 GHz is demonstrated.