In this paper, a novel microstrip-to-microstrip vertical transition designed with slotline stepped-impedance resonator (SIR) is presented. By substituting the proposed slotline SIR for the conventional uniform slotline resonator, a wideband filtering response with extended upper stopband for our proposed vertical transition can be realized. To verify our design expectation, a two-layered wideband vertical transition operating at 3 GHz is simulated, fabricated and experimentally tested. Good agreement between simulation and measurement results is achieved. The proposed transition with insertion loss better than 2-dB is measured from 1.64 GHz to 4.02 GHz, meanwhile an extended upper stopband from 4.78 GHz to 16.75 GHz with suppression higher than 12.1 dB is obtained.