Peroxynitrite (sometimes called peroxonitrite) is an ion with the formula ONOO⁻. It is an unstable structural isomer of nitrate, NO₃⁻. Although its conjugate acid is highly reactive, peroxynitrite is stable in basic solutions. It is prepared by the reaction of hydrogen peroxide with nitrite:

H₂O₂ + NO₂⁻ → ONOO⁻ + H₂O