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In this paper, a novel wideband differential bandpass filter (BPF) with slotline surface plasmon polaritons (SPPs) is proposed. It consists of a pair of differential microstrip-to-slotline transition, a slotline-to-SPP transition with subwavelength gradient grooves, and a slotline SPP waveguide, which supports the TEM, TE, and SPP modes, respectively. The dispersion and transmission characteristics of the proposed hybrid SPP structure are analyzed under differential-mode (DM) and common-mode (CM) operations and applied to design wideband differential filter. To validate the design concept, a filter sample has been designed, fabricated, and measured. Both simulated and measured results prove its good performance including a wide 3-dB fractional bandwidth of 147.6% centered at 4.2 GHz (f₀), a wide DM upper stopband to 4.8 f₀ (S₂₁^dd <; -30 dB), and a wide 40-dB CM suppression from 0.1 to 20 GHz.