This paper presents a 100-GHz vector-sum differential phase shifter in 28-nm CMOS FDSOI. Gilbert-cell type active switches are used to generate 0° and 180° out of phase signals and a 90° hybrid for obtaining I-Q signals. The hybrid is a compact capacitive loaded slow-wave differential branch-line coupler which is 75% smaller than a conventional 90° hybrid. The design has an area of 0.565 mm2 including RF and DC-bias pads with the total current consumption of 122.9 mA from a 1-V supply. The phase of the RF signal varies from 45° to 315° with a resolution of 90°. Measurements at 100 GHz show that the root-mean-square (RMS) gain and phase error are 1.5 dB and 13°, respectively. The input matching is better than -20 dB and the output matching is around -9 dB at 100 GHz. After redesigning the layout, a minimum RMS phase error of 1.7° is expected by simulations.