A novel design for high gain and low radar cross section (RCS) waveguide slot antenna is proposed. Low RCS screen composed of orthogonal array of complementary split ring resonator etched square patch artificial magnetic conductor (CSRR-AMC) is exploited on the antenna for radiation improvement and broadband RCS reduction. Measured results demonstrate that the operating bandwidth is increased by 100 MHz and the gain is enhanced by 3.2 dB with the proposed structure. Meanwhile, 10 dB RCS reduction is achieved over the range of 5.52–6.63 GHz, implying 20% relative bandwidth.