A class of ultra-wideband microstrip-line bandpass filters with improved out-of-band behaviours is presented by using an electromagnetic (EM) bandgap embedded multimode resonator. At first, the two EM bandgap structures are constructed by periodically loading uniform or non-uniform open-circuited stubs. Their performances are numerically characterised, aiming at exhibiting the existence of a bandstop in a frequency range of 15.0–30.0 GHz. These bandgap structures are then internally embedded in the middle part of an open-ended microstrip line resonator, constituting a modified type of multimode resonator. In the design, the dimensions of this resonator need to be properly adjusted towards allocating the first three resonant modes in the 3.1–10.6 GHz ultra-wide passband. To achieve a preferably wide upper stopband, the transmission zero of parallel-coupled lines is used to cancel the spurious fourth-order resonant mode. Also, higher-order modes in the upper band are inhibited by the employed bandgap structures. Two bandpass filters using the nonuniform stub-loaded resonator are finally designed, fabricated and tested. Predicted insertion/return losses and group delays are experimentally confirmed in a wide frequency range of 1.0–30.0 GHz.