A millicharged neutrino propagating through magnetic fields experiences a small Lorentz-force deflection, which induces a geometric time delay. In the ultrarelativistic regime relevant for supernova neutrinos, this delay scales as $q_{\nu }^2{E}_{\nu }^{-2}$, where $q_{\nu }$ and $E_{\nu }$ denote the neutrino millicharge and energy, respectively, and thus shares the same leading energy dependence as the standard time-of-flight delay induced by neutrino mass. Motivated by this similarity, we propose a framework to reinterpret supernova time-of-flight limits on neutrino mass as constraints on neutrino millicharge. We express both effects in terms of a common $E_{\nu }^{-2}$ dispersion coefficient and compute the millicharge-induced contribution using a line-of-sight-dependent magnetic delay kernel, extending the original SN1987A uniform-field estimate. Applying this translation to existing SN1987A limits and to projected sensitivities for future Galactic core-collapse supernova observations, we obtain bounds ranging from the $\sim {10}^{-17}e$ level for SN1987A to the low-${10}^{-19}e$ regime for next-generation Galactic bursts, with optimistic combinations of detector sensitivity and Galactic sightline approaching $\sim {10}^{-20}e$. We compare these results with other bounds in the literature and discuss how nonzero neutrino mass affects the interpretation.

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• Extrasolar neutrino astronomy
• Neutrino interactions
• Novae & supernovae
• Neutrinos