With the increase in wireless services, the demand for antennas that can operate at more than one frequency has increased. This work proposes a slot antenna whose frequency of operation can be configured into three bands of 2.4 GHz, 5 GHz, and 3.5 GHz for the Wireless-Area-Network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX) applications. The switching between the three bands is achieved by two PIN diodes properly placed between the two sides of the slot. The antenna consists of a rectangular slot etched on the ground plane, while on the other side of the substrate, there is a microstrip line to feed the slot with an open stub for matching. The tri-band frequency reconfigurable slot antenna has been studied, and its parameters optimized using computer simulation Technology (CST-MWS). Parametric study on the slot dimensions and the microstrip feeding is presented. For verification of the simulation results, the antenna is fabricated and measured. The simulated and measured parameters such as return loss, radiation pattern, and gain show excellent agreement for the three operation bands.