An ultrawideband RF photonic phase shifter implemented using two cascaded polarization modulators (PolMs) is proposed and experimentally demonstrated. The first PolM (PolM1) is employed to generate an optical carrier and two first-order sidebands that are orthogonally polarized. By aligning the polarization directions of the optical carrier and the two first-order sidebands from the first PolM with the two principal axes of the second PolM (PolM2), complementary phase modulation on the optical carrier and the first-order sidebands is produced if a DC voltage is applied to the second PolM. By selecting the optical carrier and one sideband using an optical filter and beating them at a photodetector, an RF signal with its phase that is tunable by tuning the applied DC voltage is generated. The proposed phase shifter has the advantages of fast tuning and an ultrawide frequency tunable range. An experiment is performed. A full 360 degrees phase shift over an ultrawide frequency range from 5 to 40 GHz is demonstrated. The spurious free dynamic range of the phase shifter is also studied.