An innovative vector-sum phase shifter with a full 360° variable phase-shift range in 0.18-μm CMOS technology is proposed and experimentally demonstrated in this paper. It employs an I/Q network with high I/Q accuracy over a wide bandwidth to generate two quadrature basis vector differential signals. The fabricated chip operates in the 2.3-4.8 GHz range. The root-mean-square gain error and phase error are less than 1.1 dB and 1.4° over the measured frequency span, respectively. The total current consumption is 10.6 mA (phase shifter core: ~ 2.6 mA) from a 1.8 V supply voltage and overall chip size is 0.87 × 0.75 mm2. To the best of the authors' knowledge, this circuit is the first demonstration of microwave CMOS phase shifter with very low phase error over a wide bandwidth employing the vector sum method for all monolithic microwave integrated circuit phase shifters with 360° phase-control range to date.