ABSTRACT 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 × 10 24 and 11.8 × 10 24  m⁻ 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 NH 4 + and NO 3 ⁻ accumulation, suggesting that oxidative flux was partially diverted toward nitrogen conversion, thereby competing with carbon mineralization.