Abstract Lorentz invariance is a fundamental symmetry of spacetime and foundational to modern physics. One of its most important consequences is the constancy of the speed of light. This invariance, together with the geometry of spacetime, implies that no particle can move faster than the speed of light. In this article, we present the most stringent neutrino-based test of this prediction, using the highest-energy neutrino ever detected to date, KM3-230213A. If we assume an extragalactic source as the origin, the arrival of this event, with an energy of $$220_{-110}^{+570}\text{mbox}PeV$$<math><mn>22</mn><msubsup><mrow><mn>0</mn></mrow><mrow><mo>-</mo><mn>110</mn></mrow><mrow><mo>+</mo><mn>570</mn></mrow></msubsup><mspace></mspace><mtext>PeV</mtext></math>,setsaconstrainton$\delta \equiv {c}_{\nu }^{2}-1\, &lt; \,4.{2}_{-3.7}^{+9.2}\times 1{0}^{-22}$ $\delta \equiv c_{\nu }^2-1<4.2_{-3.7}^{+9.2}\times 10^{-22}$ .