In this paper, a novel hybrid multimode resonator (MMR) is proposed and applied for design and exploration of a wideband filtering balun with the functionality of vertical transition. This hybrid MMR consists of two distinctive kinds of transmission lines: 1) microstrip line placed at the top and bottom layers, and 2) slotline formed on the middle common ground. With this combination, a hybrid MMR is constituted not only to realize two wideband filtering baluns, but also to achieve wideband vertical transition function. By virtue of the field conversion from the slotline to the output coupled lambda/2 microstrip line resonator, the 180 out-of-phase and equal magnitudes at the balun's outputs can be achieved inherently over a wide frequency range. Meanwhile, multiresonant modes can be excited by properly setting a certain offset distance at the two feeding points in the hybrid MMR. As such, good signal transmission and wideband filter performances between the topand bottom-layer microstrip lines can be gained over a desired wideband frequency range, and they can be determined by proper reallocation of these resonant modes. As examples, two prototype circuits are designed and fabricated to confirm the proposed concept. Furthermore, both simulated and measured results indicate that this proposed balun filter exhibits not only a high-selectivity filtering performance, but also good phase and magnitude imbalance performance in the wide operating band with the functionality of vertical transition.