In this paper, a novel tunable microstrip monopole antenna using defected ground structure has been presented. The configurability of the antenna is implemented by a tunable matching network comprising double U-shaped defected ground structure with islands. The frequency agility is thus easily realized by capacitive loading onto the islands. Simulation and measurement results of a prototype monopole antenna verify the frequency tuning from 2.7 GHz to 2.1 GHz at voltage variation of 29 V to 5 V. Instantaneous bandwidth of this prototype is measured from 65 MHz to 15 MHz. Good matching of more than 21 dB as well as monopole like antenna pattern has been achieved over tunable frequency range of 623 MHz.