A new approach for in-band radar cross section (RCS) reduction of microstrip patch antenna with preserved radiation features is presented. This aim is achieved by the application of highly absorbing unit cells optimized by particle swarm optimization around the radiating patch of the reference antenna. More than 10 dB in-band RCS reduction is observed from 2.5 GHz to 3 GHz for both x- and y-polarizations with a -26.4 dB maximum RCS reduction at 2.8 GHz, according to measurement results. A close agreement between the measured and the simulated results shows that the proposed absorbing unit cell is a good candidate to enhance antenna in-band stealth performance. In addition, with a comparative table, it has been shown that the proposed antenna has much lower in-band RCS and is more compact than the other antennas proposed in previous literature. The basic benefit of this study is to achieve maximum in-band RCS reduction, especially at the exact desired frequency without long-term simulations and time-consuming design studies.