In this article, a compact wideband microstrip-to-microstrip (MS-to-MS) vertical transition designed with a slotline stepped-impedance resonator (SIR) is first presented. Compared with the existing wideband transitions, this proposed transition centered at f₀ can tremendously extend its upper stopband via two introduced transmission zeros around 3f₀ and 5f₀. With the designed equivalent circuit, the working principle is theoretically discussed. To realize the size compactness of this proposed wideband transition, the slotline SIR is replaced by two back-to-back connected slotline Y-junctions. Finally, a prototype wideband transition is simulated and fabricated. A wideband filtering response with its upper stopband up to 6.06 GHz under attenuation better than 15 dB is experimentally achieved as expected in the simulation.