In this paper, a new type of device called dual-band coupled-line bandpass balun filter is presented. Based on the traditional coupled-line filter theory and Marchand balun configuration, a new device with both filter-type, as well as balun-type characteristics is proposed. The new device utilizes -type transmission-line stepped-impedance resonators to achieve a dual-band operation. Besides providing a simple design procedure for the device, its working mechanism is also revealed mathematically. A prototype balun filter operating at 2.4 and 5.8 GHz has been realized using traditional printed circuit board technology to validate the proposed concept and theory, showing promising application potentials for future multiband RF wireless transceiver modules. Experimental measurements show good agreement with analytical and computer simulations.