This paper presents the design of a phase shifting impedance tuner for the terahertz photomixer arrays. The proposed impedance tuner uses the triple stub circuit topology that allows the control of the input impedance and insertion phase simultaneously. The impedance tuner targets to tune the photomixer antenna input impedance, which is commonly in the range of 70-to-200., to the photoconductor input impedance, which can be as high as 10 k., reducing the significant impedance mismatch and increasing the radiated terahertz power. The proposed tuner can also control the insertion phase without disturbing the impedance tuning. The simulation results show that the proposed circuit can have an input impedance between 500 Omega to higher than 10 k., while it can change insertion phase from -45 degrees to -155 degrees at 340 GHz. The proposed tuner can overcome the impedance mismatch problem in photomixers and be used as a phase control circuit for beam steering photomixer arrays.