A new ring wideband bandstop filter (BSF) and its exact design method is proposed and investigated in this letter. The BSF is devised by introducing an additional transmission line to a traditional hairpin bandstop filter in parallel, hence giving enhanced selectivity and attenuation performance. A lumped lowpass equivalent circuit is introduced to explain the mechanism of the filter. Rigorous correspondence between the BSF and its equivalent circuit is established by comparing their even-and odd-mode input admittances. This enables the BSF to be synthesized the same way as lumped filters. Both the BSF and the method have been documented by a design example.