A multi-input multi-output (MIMO) antenna system is presented for wireless devices operating at WLAN (2.45, 5.25, and 5.775 GHz) bands. Each of the two antennas in the MIMO system consists of a crescent-shaped monopole whose first part covers the 2.45 GHz band while its second part covers the 5.25 GHz and 5.775 GHz bands. The second part of the monopole is a slot etched in the protruded ground plane between the two antennas. A decoupling mechanism in the form of two interlaced ring-shaped slots is used. The proposed MIMO antenna system is designed on an FR4 substrate with overall dimensions of 40x47.5x1.5 mm and a small edge-to-edge spacing of 7.3 mm between two antennas. According to the measured results, the proposed design covers two frequency bands (2.2-2.83 GHz and 5.03-5.95 GHz) and has a mutual coupling of -20.78 dB at 2.45 GHz and -42.65 dB at 5.55 GHz. The proposed antenna's performance in both simulations and testing indicates that it is a good choice for WLAN applications.