A circularly polarized antenna array for the automotive radar frequency band at a center frequency of 79.5 GHz is proposed. The antenna array consists of 10 dual-linearly polarized square microstrip patch elements in 50 Ω series-feed configuration. Amplitude tapering is applied in order to reduce the sidelobe level. The antenna array is fed from a WR10 rectangular waveguide and a transition which transforms the waveguide mode into a microstrip line mode. A 3 dB power splitter and a phasing line are used to feed the two channels of the dual-linearly polarized antenna array with equal magnitudes and 90° phase offset. As a result, the antenna radiates a circularly polarized wave over the complete frequency band from 78 to 81 GHz with an axial ratio in boresight direction of lower than 3 dB.