A class of wideband microstrip-to-microstrip vertical transitions is proposed via multiresonant modes in a slotline resonator formed on their common ground plane. The slot acts as a multimode resonator with microstrip lines at its two sides as the input and output terminals. Multiresonant modes in this slotline resonator can be properly excited by setting a certain offset distance at two feeding points. As such, good signal transmission between the top- and bottom-layer microstrip lines can be achieved over a wide frequency range determined by these resonant modes. In this context, the initial transmission line models of these transitions are further deduced. Then, their simplified generalized high-pass topologies are synthesized to design these transitions with prescribed operating bandwidths and ripple levels. Finally, by calculating the turn ratios of those involved transformers in the initial transmission line models of these transitions, the synthesized models are constructively mapped into all the physical dimensions to allow one to carry out an efficient design of these transitions. As examples, two vertical transitions are designed, fabricated and tested to verify the predicted frequency responses. The synthesized, simulated, and measured results are found in good agreement with each other.