In this letter, a wideband balun bandpass filter (BPF) composed of a triple-mode slotline resonator and a couple of microstrip-slotline cross-junction is presented. The proposed triple-mode resonator consists of a half-wavelength (lambda/2) uniform impedance resonator (UIR) and two different short-circuited slotline stubs installed in middle of slotline UIR. The first three modes of the cross-shaped slotline resonator are then excited and utilized to achieve the balanced-to-unbalanced passband. By tuning the short-circuited stub length, its controllable bandwidth can be realized. Owing to the intrinsic 180 degrees phase difference at two output ports of the microstrip-slotline cross-junction, a wideband passband is achieved with improved magnitude balance and out-of-phase properties. To validate our design concept, a wideband balun BPF operating at 4 GHz with 0.6-dB magnitude imbalance and 8 degrees phase imbalance is designed and fabricated. Both simulated and measured results are found in good agreement.