Mid-infrared plasmonic resonance enables nanoscale light confinement at mid-infrared frequencies, leading to various applications ranging from compact infrared lasers to biological and chemical sensing. However, upon fabricated, plasmonic resonators normally have a fixed resonance frequency, which limits their application frequency range and hinders the dynamic tuning potential. Here, with the flexible PEDOT:PSS ((poly(ethylenedioxythiophene):poly(styrenesulfonate)) conducting polymer as the plasmonic medium, highly tunable and reversible mid-infrared plasmonic resonances are demonstrated via mechanical stretching. Such plasmonic resonances, based on the stretching-induced grating-type morphology of the PEDOT:PSS thin-film, can be readily tuned across a large mid-infrared frequency range from ≈7500 to 1500 cm