A slow-wave half-mode substrate integrated waveguide with a spoof surface plasmon polariton (SW-HMSIW-SSPP) transmission line structure is proposed, analyzed, and demonstrated. Results show longitudinal and lateral size reductions of more than 80% and 73%, respectively, as compared with an equivalent SIW at the same cutoff frequency while maintaining a low attenuation constant of 0.02 dB/mm. By altering the geometric parameters of the SW-HMSIW-SSPP the dispersion curve can be tailored. The dispersion and electromagnetic (EM) field distribution characteristics of the proposed SW-HMSIW-SSPP unit cell and overall structure are analyzed by HFSS simulations and an equivalent circuit, and pertinent performance parameters are compared to those of SW-HMSIW and HMSIW-SSPP structures. A proof-of-concept PCB SW-HMSIW-SSPP sample was fabricated, and measurement results showing a band-pass of 4.8-11.5 GHz are found consistent with simulation results. The SW-HMSIW-SSPP structure has applications in the design of miniaturized microwave circuits.