Tetramethylammonium hydroxide (TMAH), a toxic and persistent compound widely used in semiconductor manufacturing, was degraded using underwater plasma at applied fields ranging from 0.8 to 10 kV mm⁻1. Optical emission spectroscopy revealed OH(A–X), Hβ, and O I features, with Stark broadening of Hβ indicating electron densities between 5.0 × 1024 and 11.8 × 1024m⁻3, characteristic of high-field discharge. Spectral analysis showed a field-dependent shift from OH-dominated to O-dominated chemistry. Total organic carbon (TOC) removal followed pseudo-first-order kinetics, with rate constants increasing from 3.6 × 10⁻3 to 7.7 × 10⁻3 min⁻1 but plateauing above 4.8 kV mm⁻1. Ion chromatography detected NH4+ and NO3⁻ accumulation, suggesting that oxidative flux was partially diverted toward nitrogen conversion, thereby competing with carbon mineralization.