In this paper, a novel, compact, ultra-wideband tuning, concentric pentagonal slot-line-based frequency reconfigurable multipleinput- multiple-output (MIMO) antenna system is presented. It consists of a 4-element antenna design on a commercially available FR-4 substrate with dimensions 60×120×1.56 mm3. All the antenna elements are planar in structure that are etched out from ground (GND) plane. Frequency reconfigurability is achieved using varactor diodes by reactively loading the antennas. Each capacitance value of the varactor diode results in triband operation. Proper capacitive reactance loading enables the antenna to resonate over a wide frequency band. The proposed MIMO antenna system is tuned over an ultra-wide frequency band and a smooth variation of the resonance frequencies is observed from 1.32~1.49 GHz and 1.75~5.2 GHz. Moreover, the unique feature of the proposed antenna system is the effectiveness of reactive loading the slot over all the resonating bands. The proposed antenna system is compact and suitable to be used in wireless handheld devices and mobile terminals for cognitive radio (CR) applications. The envelope correlation coefficient (ECC) did not exceed 0.186 in the entire operating band of the MIMO antenna operation. The maximum measured gain of the MIMO antenna is 4.5 dBi with maximum efficiency of 81%.