A novel wideband bandpass filter using back-to-back microstrip-slotline transition and triple-mode slotline resonator is proposed. The triple-mode resonator is constructed with one full-wavelength slotline ring and a single short-circuited stub. Two microstrip feed lines are placed above the slotline in such a way that two intersection positions are placed at the 54° separation along the ring. With a proper choice of the length of stub, the first three resonances can be quasi-equally located to construct a wide passband. To verify the design concept, a filter prototype is finally designed, fabricated and measured. The predicted results are well confirmed in experiment.