A triple-band metamaterial absorber (MMA), which is ultrathin and polarization-insensitive, is designed and fabricated. The top layer of the MMAs consists of a square-shaped copper film with a cross cut out in the center. The calculation result demonstrates that there are three distinct absorption peaks, which are 5.86, 15.04, and 17.5 GHz, and their corresponding absorption rates are 99.76%, 99.11%, and 99.90%, respectively. Moreover, the thickness of the MMA is very thin, only 1.05% of the wavelength of the lowest absorption frequency. The MMA also shows polarization-insensitivity for normal incident wave. In addition, we analyzed the absorption characteristics of the three absorption peaks and the impact of key structure parameters on the MMA’s absorption. Through experiments and theories analysis, it can be seen there are small absorption errors and frequency deviations of three absorption peaks. The MMA is ultrathin and has simple structure and perfect absorption, and it can be widely applied in many fields, such as detection sensors, imaging, and thermal radiometer.