A 79-GHz variable-gain vector-sum phase shifter is presented, which consists of 5-bit digital attenuators in respective I and Q paths. It is designed to represent any vectors in the entire IQ plane by controlling the digital attenuators with low phase variations. Variable-gain function can be achieved intrinsically without additional variable-gain circuits. The proposed phase shifter can maintain low phase error at high input powers. The phase shifter provides a root mean square (RMS) phase error of 6.74 degrees with a phase resolution of 11.25 degrees. The gain dynamic range and resolution are 21 and 3 dB, respectively, with an RMS gain error of 1.89 dB.