A planar dual-band monopole antenna with a frequency-tunable band is presented. The structure of the antenna radiator has a stem connecting to two branches that are used to generate two frequency bands at around 2.4 and 3.4 GHz for Worldwide Interoperability for Microwave Access (WiMAX) applications. The lower band covers the WiMAX frequency band of 2.3-2.4 GHz, while the higher band is frequency-tunable to the WiMAX frequency bands of 3.3-3.4, 3.4-3.6and 3.6-3.8 GHz. The frequency tunability is achieved by using the reverse-bias voltage across a varactor that is placed between the stem and one of the radiating branches of the radiator. In this study, the radiating branch responsible for the higher band is selected for tuning. A simple and novel biasing circuit, consisting of two radio frequency (RF) choke resistors and an L-shaped stub, is designed for biasing the varactor. Results show that the higher band can be continuously tuned in frequency, yet keeping the lower band unchanged. The reflection coefficient, radiation pattern, and efficiency of the antenna are studied using computer simulation and measurement.