In this work, a system for measuring moisture content in bundles of recycled cardboard sheets using microwaves is proposed and evaluated. For the development of this system, a microwave generator and an antenna tuned in the X-band were used. The detector system used was formed by a diode array point of contact. Calibration curves of moisture content in bales versus voltage in the array were obtained for two orientations or positions of the cardboard sheets in the sample bales. The results showed that the higher the moisture content, the lower the voltage is induced in the diodes. It was also observed that the attenuation of the microwave signal depends on the position of the cardboard sheets and that the measurement can be made in samples of bales with a maximum of 35% humidity, because on this percentage, the variation of voltage in the diode array is almost zero.