We propose a supercompact and ultrawideband (UWB) bandpass filter for spoof surface plasmonic polariton (SSPP) mode. By using meander-line technology, the electrical size of the proposed SSPP structure is reduced dramatically, which is only about 1/4 of the traditional rectangular-groove structure with the same geometry. Based on the proposed SSPP structure, we construct a supercompact UWB bandpass filter working in the frequency range of 0.25-4.5 GHz, whose geometric size is only 87.5 × 12.8 mm 2 . The upper cutoff frequency of the passband can be controlled by changing the meander-line design of the SSPP unit, and the lower cutoff frequency of the passband is determined by the capacitive coupling of the feeding structure. Both the simulation and experimental results show that the UWB SSPP bandpass filter is centered at 2.4 GHz with an extremely wide relative bandwidth of 174%, whose electrical size is only 0.7λ × 0.1λ at the central frequency.