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In this letter, we report the design theory and analysis of an end-coupled broadband spoof plasmonic based band-pass filter (BPF) at sub-wavelength scale enabled by multi-mode grooved stepped impedance resonators (GSIRs) at THz frequency regime. The proposed GSIRs are end-coupled to each other and externally coupled to the source and load through spoof surface plasmon polaritons (SPP)-based transmission line. The designed filter has 3-dB bandwidth of 0.35 THz (from 0.17 THz to 0.52 THz) and fractional bandwidth (FBW) of 87.85%, respectively, with the shape factor of 1.35. The simulated transmission and reflection coefficients in the pass-band of proposed filter are better than ∼6 dB and 15 dB respectively. Also, 60 dB spurious rejection bandwidth of the designed filter in the upper stop-band is 1.3 THz (0.62–1.92 THz). For the validation of the proposed approach, similar design is implemented at microwave frequency due to the geometrical dependence feature of spoof SPP and its S-parameters response is presented. Such broadband BPF will pave a path to design and development of advanced integrated plasmonic circuits and systems for high-speed and low-loss THz wireless communication applications. [ABSTRACT FROM AUTHOR]