Abstract We present a JWST mid-infrared (MIR) spectrum of the underluminous Type Ia Supernova (SN Ia) 2022xkq, obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) ∼130 days post-explosion. We identify the first MIR lines beyond 14 μ m in SN Ia observations. We find features unique to underluminous SNe Ia, including the following: isolated emission of stable Ni, strong blends of Ti ii , and large ratios of singly ionized to doubly ionized species in both Ar and Co. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the Co iii 11.888 μ m feature and the SN light-curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements, we constrain the mass of the exploding WD. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD ( M WD ≈1.37 M ⊙ ) of high central density ( ρ c ≥ 2.0 × 10 9 g cm −3 ) seen equator-on, which produced M ( 56 Ni) =0.324 M ⊙ and M ( 58 Ni) ≥0.06 M ⊙ . The observed line widths are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of subsonic carbon burning followed by an off-center deflagration-to-detonation transition beginning at a single point. Additional observations may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.